Fujichrome Provia and Velvia are just about it, these days.
AgfaPhoto CTprecisa 100 Color Transparency Film is another film you might try. It's supposed to have "faithful, natural color reproduction", whatever that means.
Check out B&H's stock (
http://www.bhphotovideo.com ).
I feel your pain. I was a multi-image (high tech slide show) producer in the 1980s. I shot tens of thousands of slides on Kodachrome 64, Ektachrome 64 (both Daylight and 3200K), Ektachrome 400, Ektachrome 5071 and SO-366 duplicating films, Kodalith, and the Fujifilm stocks listed above.
Unfortunately, digital imaging processes have long since surpassed film in almost every respect. I still have all my film cameras, but haven't loaded a roll of film in any of them since January, 2005.
Most people copying flat art in museums, schools, and industry use digital cameras, and print archivally on Epson printers, using Ultrachrome inks and fine art papers.
With proper color management, digital copying results are astoundingly close to the original... certainly better than I ever got with film, and I had the best equipment available when I was doing copy work.
There is almost no variability from day to day, week to week, month to month, in such a digital process. With E-6 processing, you're at the mercy of the lab's precision in maintaining process control on any given day (heck, okay, from batch to batch!). A digital file printed on the same printer a month from now, using the same inks and paper stock, will be virtually indistinguishable from a print made today.