Only ProMediaGear appears to offer an L-bracket for 90D with grip ($150).
https://www.promediagear.com/PLCBGE14T-L-Bracket-for-Canon-90D-80D-70D-with-BG-E14-Grip-for-Tilt-Screen-Arca-Swiss-Type-L-Plate_p_440.htmlHowever, since the 90D uses the same BG-E14 grip as 80D and 70D, there may be another two-step solution.
First, get a "fitted" anti-twist plate for the bottom of the BG-E14. There are several possible sources of these:
Acratech ($40)...
https://www.acratech.net/camera-plate-for-canon-bge7-bge9-bge11-bge14-and-more/ProMediaGear ($20)....
https://www.ebay.com/itm/Promediagear-Bracket-For-Canon-70D-PBC70D-Arca-Swiss-RRS-Free-Shipping/264881553058?hash=item3dac2bbea2:g:SiEAAOSwjJFfc46k or
https://www.promediagear.com/PCBGE13-Canon-90D-80D-70D-BG-E14-Plate-Arca-Swiss-Type_p_155.html There may be other fitted bottom plates available for the BG-E14. Those are just what I found with a quick search. (Note that the ProMediaGear plate shown on eBay is not the same as the one used with their bulkier, modular L-bracket kit ($150). Apparently they have had two or three different versions.)
In most cases DO NOT get one of the "universal" plates
without the anti-twist features designed to fit the bottom of the grip. Those are important so that the plate can't accidentally turn and loosen itself. (Note: Rubber pads on an Arca plate
do not serve very well as an anti-twist feature. They may help, but won't prevent it turning & loosening at times. You need an actual lip or tabs or set screw or similar that positively prevents the plate from turning. [i]Some universal/generic plates or L-brackets have these... see note about Sunway and link below... but many do not.)
Once you have one of those bottom plates, add a "universal" modular L-bracket:
Kirk Photo offers a "generic" L-bracket that can attach directly to the bottom plate.
https://www.kirkphoto.com/camera-mounting/canon/eos-90d-with-bg-e14/universal-quick-release-tall-l-bracket.html Acratech offers two different "generic" L-brackets, a "standard" and an extra long version:
https://www.acratech.net/arca-style-universal-l-brackets-by-acratech/ and
https://www.acratech.net/l-brackets-and-nodal-rails/extended-universal-l-bracket/I don't have any personal experience with these products on a 90D (or 80D, 70D, for that matter) with BG-E14. The Kirk and Acratech L-brackets will almost certainly need to be removed to access the ports on the left side of the camera or to change batteries in the BG-E14. They also might interfere with folding out the camera's rear LCD screen. At least with both these L-brackets removal is easy... just a thumbscrew (no tools needed). Some of the other modular style L-brackets require an Allen wrench or other tool.
The ProMediaGear modular L-bracket (I notice it also serves as a flash bracket,... that's neat!), extends to the side allow access to the ports on the side of the camera and appears to at least allow the rear LCD screen to be extended to the side, although it would block reversing it to view from the front "vlogger" style. But I'm pretty certain it too will need to be removed whenever you need to change out the batteries in the grip. I don't know if tools are needed to remove it.
Finally, Sunway makes a number of different "universal" L-brackets
with built-in anti-twist feature that are relatively inexpensive.
http://www.sunwayfoto.com/e_goodsmulu.aspx?cid=49 Some of those are modular, others are not. Maybe one of them would be a good fit for 90D with BG-E14. They have detailed dimensions for each bracket at that web page that might help you choose one to fit your camera and grip. Of course, these will also require removal (or partial disassembly, if modular) to access the camera's ports and change the batteries in the grip.
Don't give up! Watch eBay carefully. I was shopping for a fitted Arca-style bottom plate for a Canon M5 mirrorless recently and found only two being offered in US for it... both rather pricey for a simple plate. After searching for a while longer I found found one on eBay for all of $16 including shipping. It's actually a modular L-bracket, too. I can always remove the end piece to use it simply as a bottom plate. For so little money I was expecting a long wait on shipping and a fairly crude, but hopefully usable product. I was very pleasantly surprised to find it in my mail box within a week and that it's really very nicely finished and a very good fit! I'm really not a fan of the bulk that L-brackets add, but may keep this one configured that way because in this case it's actually low profile and pretty small (since the M5 is a small camera) and not uncomfortable to handle when shooting.
Since then I tried to find that same product again on eBay (to recommend it to someone) but haven't seen it advertised. I did find some other similar, slightly more expensive ones to fit M5. My point is that there may be something show up there that for your 90D and BG-E14. But stuff comes and goes on eBay. And much (most?) of the stuff from China works out pretty well I've bought various other things... tripod head adapters, macro extension rings, lens adapters... all have been well made and promptly delivered.
That's not always the case, though. I recently took a chance and ordered two circular polarizing filters from China and am still awaiting them over three weeks later. The tracking says they're sitting in customs on one side or the other of the Pacific. It might be because they were shipped from Wuhan, China adding some causes for concern! (Normally I have pretty high standards for filters... But for a total of $35 incl. shipping for 43mm
and 72mm size C-Pols that are supposedly made with German Schott glass and Nano multi-coated, I figured what the heck, they're worth a try!)