When I made botas a lot, I used various leathers for various styles, wants, and time periods:
1) Based on originals I've examined the most common leather used was brain tanned buckskin - these were decorated in various style including: Indian and Mexican - some of the Mexican were "tooled" by using heated brass plates with the deco engraved on it and then pressed into the damp leather. The only downside to brain tan is the cost - an excellent alternative that looks like brain tan is the German Tan available from Crazy Crow, you can even get it smoked. The only downside to the German tan is it's a bit heavier than buckskin, being thicker and thus heavier like elk hide. Another lighter weight alternative are the real chamois cleaning cloths from auto parts houses.
When using any type of "soft" buckskin, elk hide, or similar no matter the tannage , I highly recommend to pre-stretch it.
2) 2-3 oz Bark tan cow hide can also be used albeit the finished botas will be firmer/stiffer
3) I have also used the sheepskins from Crazy Crow - they make up into a super nice pair, especially for those desiring a slick side on one side and they are very cost effective - pre-stretch them.
4) If historical accuracy is not a big concern at least prior to 1880, then any chrome tanned type hide will work: Elk (4-5 oz), Deer (2-3 oz), buffalo (3/4 oz), and cow hide (3-4 oz upholstery sides, buck tanned, and blacksmith side). Again soft hides like deer, elk, buffalo, etc should be pre-stretched.
FYI- while chrome tanning was discovered in the 1860's, this type leather did not become commercially available in the USA until 1880 and later.
any way I hope that helps.