Hi Jake, I'm sure there are people who want SS horizontal dropouts, you just don't see them on the forums much. When you do, many use the eccentric BB to take up the chain slack. A horizontal dropout would be a simpler, possibly cheaper, solution for those riders.
The IP-256 & IP-057 are the most popular frames right now, both have replaceable rear dropouts, they may even be the same parts for both frames. I believe they are the same for my IP-036 & IP-057. If you are interested in machining drop outs, you don't need a whole frame, you could get a set of thru axle drop outs and get the size, shape, and mounting from there.
If you still want to order a frame, XMIplay.com is very popular from many members on this forum. However, there are many sellers on eBay is common as well.