We have found that dacron interfacing from the fabric store works ok. Not great, but OK. Select an interfacing that has a good surface sheen to it. It releases much better than the material that is kind of dull finished.
I have spent hours trying to pick off peal ply a square inch at a time when it was the dull-finished stuff. The "official" peel ply that you buy at composites supply stores does release better.
Pro tip: if you are having trouble getting peel ply to peel, you can warm the surface locally with a heat gun and it peels right off. Be very careful though, you can scorch or warp a part by overheating it. A hair dryer would be a safer option, but might not get it locally warm enough. Haven't tried that.
Composites supply places: (in no particular order of preference, we tend to shop around for what we need)
Composite Envisions
US Composites
Soler Composites
ACP Composites
Fiberglast