Dollar Tree has started adding more cheap craft products to their stores. They stock PRIMER which is not a bad yarn. They have a pretty good selection of colors, but the skeins are pretty small. But for about $1 each its totally worth it!
If you’re from the uk places like aldi and even the range sell yarn for like a pound or a bag of 2+ yarn balls for a couple pounds. That’s where I get mine, unless I need a specific type of yarn I’ll have to shop at hobbycraft where it’s a bit more pricey. Otherwise best bet is probably online cuz everything’s cheaper there
I'm not sure how realistic this is, but I got a huge bag of yarn from an estate sale, so you could search there. No saying you'll get exactly what you want/need but it could be fun to start some random projects!
I get my cheapo yarn from Michaels, they're a big craft supplies chain store kinda like Hobby Lobby minus the shitty parts about HL. I think all of their yarns contain some levels of synthetics though in case that could be an issue (dunno how many of them if any contain biodegradable synthetics, everything I've bought has contained varied levels of acrylic)