All depends on what you want really.. you can find really lovely and cute teddies in charity shops . If you in States maybe thrift stores? I have to stop myself going into charity shops in UK because I don't live alone and I'm secret with my love of plushie etc.. but I could get loads in those shops... Including cute outfits....
Definitely recommend checking thrift shops! You can find some really great plushies there. I've gotten a lot of cute, good quality plushies from my local one for less than $5 each! They're hit or miss for sure tho. Helps if you can ask when they put out new inventory so you can plan your trips accordingly tho!
Also always always always was thrifted plushies as soon as you get them home. Thift shops are usually good about making sure their donations don't have any unwanted insect hitchhikers but I always do it any way, because better safe than sorry!
I'd definitely recommend thrift stores and places like Walmart/Target for plushies! I know they have little corners for Pokemon plushies if that's what you're looking for specifically! Thrift stores can often have some neat things, especially big-city ones!
If you don't mind purchasing things online (and paying for shipping), you can likely find plushies on places like eBay or Mercari. I definitely got a growing collection of Glaceon plushies thanks to Mercari but that can definitely be on the more expensive side. (Though, Mercari does give you an option to haggle below the asking price the seller wants. So who knows, maybe you'll strike a deal!)
Definitely wishing you luck in finding some playmates for your Eevee plush~