I'd just like to note that both infinity (they are coming out with a new version, though it's going to be sold in 100ft reels, if I recall correctly) and silkworm threads are discontinued. So those may be harder to find.
Either way, I just use 100% nylon thread spools that my mom gets (she quilts), and strip the thread myself. It's quality stuff, so it's strong, and it's quite invisible when stripped. I also have a pen thread reel, so I use that (the thread that came with it has lasted thus far)...if the thread breaks, I'll just end up using the nylon thread I normally use anyway in there instead.
As far as elastic thread goes (despite that not being the topic, I'm including it for additional information), I tend to stick to Mesika's thread reels (I tie my own loops), again because the Infinity reel is no longer produced (and near impossible to find), and I've just had good experiences with Mesika's elastic thread.
