What is the official name for wiring hole stops/plugs and where can I buy them?
They are called "wire grommets" or "electrical bulkhead fittings", and if you search for those (or some combination of those words) you should find what you're after. 

I typically seal large exterior holes with expanding foam, and smaller holes with silicone caulk. I've run many cat5 and coax cables through walls, and generally keep the hole just slightly larger than the cable, and it seals up fine. You can leave some slack on both sides (I typically leave a drip loop on the outside and a coil of a few inches on the inside where it's not visible), and that's all the flexibility needed. Silicone caulk has enough give that it is fine for this application.

For a larger cable or a couple cables, I'll go through the exterior wall with a conduit, then seal around the conduit with foam and the outside with silicone (expanding foam breaks down under UV, so it's best to cover the exterior bit with some opaque caulk). At the end of the conduit I'll put a wire grommet in a small box (a 90 degree PVC pull box works great).