Most won't be worth painting, but sometimes swelling the ranks of the DM's monster minis means going with the next best thing. This time of year, that gets easier since a good amount of cheap Halloween favors will make decent minis.
Just today a $3 bag at a wallgreens netted me a lot of bugs including multiples of;
Some iffy looking plastic flies
Very good small sized plastic ants
very good small/medium plastic spiders
Decent medium rubber spider, that would have to have it's feet glued onto the base to fit
Very good, if flat, large red tinged rubber roach. It's antenna almost look like horns
Mediocre rubber centipede. Maybe wrapped around a column it could work.
Decent rubber pincer beetle with an extra set of legs
Decent huge rubber spider
Up at a Party City store I found a nice bag of Large bats with enough detail to hold a tabletop level repaint along with a bag of just barely large size rubber rats with noticable mold lines. Still the rats would look kinda nice with skaven riding them.
The huge snail is from wally world's toy isle.
Other recent finds were a bunch of glow in the dark tiny ants and flies at a Ma' & Pa' drug store. The ants will be used in a swarm base, the flies I want to convert into stirges. Also found a bag of cheap sea critters with some possibly salvageable crabs and stingrays. That store also had a boatload of plastic animals too numerous to note, and in the Halloween section they had two versions of huge rat toys that would work on the battlemat.