I typically go to the Dayton store, but Alro Steel has a place in West Chester now. The store in Dayton has a “hit or miss” offcuts selection in the little store area, but you can possibly get what you need from ordering at the counter.
I’ve bought from Queen City Polymers, also in the West Chester area. Years ago, it was not really set up for small orders, but they did get me what I needed.
Final option I’m aware of is Parts Express in Springboro, and they have Saturday hours. These are textured black or white ABS sheets, with one size rough like a hammertone paint. The other side is smooth, but might have some flow discontinuities.
In the past they had 24 x 24", and white, but that link only has 12 x 24", black, in 1/16", 3/32", and 1/8".
Just remembered: Some hobby shops have small sheets of smooth ABS, typically in white, black, and gray, but smaller pieces, like 12x12".
Just remembered: Harrington Industrial Plastics. Off exit 14, Glendale-Milford, then west a bit. I bought some oddball PVC fittings from them. They were a bit more expensive than Lowes or Home Depot, but the fittings were very high quality (plus they weren’t carried at the regular stores).