This isn’t stockinette. ST st with just a slip selvage will roll and not lay flat. I’m guessing it is a 1x1 rib pattern with slipped edges. It’s hard to tell without seeing the back side.
the 4 i’ve used the most (and i mean dozens, maybe even 200? items) are 360kool at eBay Home , kool-diy at kool-diy | eBay Stores , wiseller at eBay Home and wis-only at eBay Home .
all of them have been doing business on ebay for at least 3 years, and one of them for almost 6-1/2 years. all have satisfaction ratings of 98-99% (thousands of positives, only 2-3 dozen negatives). i haven’t had any issues with any of them.
they all sell most of their items via auction, instead of buy-it-now, so you do have to watch the items you’d like, bid, and either watch or get alerts on the listing in case you need to up your bid. i’ve yet to spend more than US$1.50 a skein, and usually in the US$0.75-1.25 range, with free shipping (with tracking).
ebay is definitely worth looking at, since you can customize searches based on fiber, girth, color, shipping, etc. good luck! 