To add to what has already been written, here's where you might have already been looking:
http://www.cooncreekoldwest.com/wildb.htm Keep in mind, these are cotton, and in the movie they wore wool (which, trust me on this, is hot as heck so I understand why they make the cotton "summer" uniforms). I have a lot of Span Am stuff made by these folks. I like their stuff and their excellent customer service, but not a big fan of the their leggings, they're pretty darned flimsy. If I did a Wild Bunch shoot, I'd go with a WW2 legging set, which are very durable and pretty easy to find (unless you have massive calves). Contrary to popular belief, the WW2-style leggings are pretty much what they had before WW1. The spiral wool "puttees" were used once the doughboys got to France. In the US, they wore leggings. The pattern was a tad bit different from the pre-WW1 era and the ones used in WW2, but it's not that big a difference, and plenty of WW2 re-enactor vendors sell good condition originals and reproductions for larger-legged folks.
If you want REALLY authentic stuff, these guys have the correct M1916 wool shirts and riding ("poofy") pants that would be correct for the movie. They also have the cavalry style M1912 holsters for the 1911, and belts and such, all very well made:
http://www.whatpriceglory.com/USList.htmAs for AEF supply, those are mostly the rougher wools you saw in France in 1917, not the finer made pre-war Cavalry uniforms in the movie.
A few things you might want to know about the movie and modern uniforms:
The campaign hat (often referred to as the "Boy Scout" or "Smokey Bear" hat) was quite different in the WW1 era than the hats the drill instructors wear today. Today's hats are greener in color, and the brims are much wider. What Price Glory makes a good repro of the original hat, but some folks probably don't care as the good reproductions cost more than the hats you can find at most surplus outlets.
Even though M1916 pullover shirts are correct for the time period, in the movie, most of them wore WW2 wool shirts. The major difference is in the buttons and the fact they button all the way to the bottom. M1916 shirts only button about 1/3 the way from the throat, and are actually "pullover" shirts.