The original supplier was Roser. However, they closed down in the early 1990s, and are no more. Sure, you can get the cheap stuff from GAHH, Word Upholstery, GAT, Cabrio, etc, and all of those sell the "Old Timer" leather which is make by Hans Reinke. As DOITWRITE pointed out, Hydesleather is where to get the Old Timer from for the best deal. I know the owner, Aaron Lathewait, personally, and is a stand up fella. His color matches are spot on, too. He actually went to the archives at the Classic center in Fellbach to match all of the colors. All other places put an asinine mark up on it, so buy from Hydes of you choose to use the Old Timer.
The Hans Reinke is considered the next best thing, and it's a beautiful representation of the original Roser leather. But if you want an authentic reproduction surface dyed hide, then you can buy the correct leather from Aeristo or Steenbuck. Aeristo in in the USA, and Steenbuck is in Europe. If buying from Aeristo, you'll want their Touranil line.
https://www.aeristo.com/products#leathers Steenbuck is here:
http://www.steenbuck-web.de/index.htm Less shipping, you'll pay around $1,500 per hide, but you get exactly what you pay for. Several years ago I did an extensive write up on the different kinds of leather compared to original leather on the M-100 forum. Here is the link:
https://m-100.clubexpress.com/content.aspx?page_id=2155&club_id=406485&item_id=830165 Sadly, the M-100 site is apparently not working, and hasn't been for a while. I tried to open the above link and it still errors out every time. Maybe you'll have better luck than I.
If any of you end up recovering the leather covered panels on the dash, around the windshield, sun visors, etc, you MUST use Speltleder, or leather that is skived almost paper thin. The edges of the door panel coverings and seat back coverings must also be shaved paper thin with a hand skiver for proper fitment. This "speltleder" is also required when making the piping that goes in the seats. About 99.9% of people that make seat covers for these cars don't know this, and the piping ends up being too large in diameter. Most of you probably won't care too much about this, but for a restoration being judged the correct diameter piping is important for authenticity.
Below are some pictures. The first one is of three pieces of leather. The top piece is an original piece of Roser leather from 1970. The middle piece is a sample from Steenbuck, and the bottom piece is a sample from Aeristo. All are wonderful examples, and the quality is unmatched.
The second and third pictures are color samples of all of the colors available from Aeristo and Hydes. Take note of the fine/smooth graining and satin sheen. If you want a picture of a particular color please reach out and I'll happily oblige. I believe Aeristo will send you small samples at no charge, but Hydes will charge you $50-$80 for a complete sample booklet, but it's worth it. I don't know if Hydes sends separate samples anymore.
Sadly, Cognac is something both have struggled with in getting the correct look. What they offer for Cognac is beautiful, but the red flecking is not like the original. Then again, if everything is done all at the same time it'll look just fine.