Not selling any, my child would murder me. So I hope you find someone here to help, if not, here are my suggestions.

Check FB marketplace. Second hand stores if you have any nearby, or places like Tuesday Morning.

Finding articulating ones cheap will be tough. Most cheap lots are solid dinos with maybe the jaw opening and closing. And even non articulating scientifically accurate ones are pricey (Safari, Papo, etc are like $20-30 each figure).

You can check Amazon. They have stuff like this: https://www.amazon.com/dp/B074GNN98J/ref=cm_sw_r_cp_api_glt_fabc_0FRCEEEWRC8MBCDRRGDT

Not the best looking, but you get a variety. They have others, it’s just an example.

Staying away from JW toys is probably a good idea. A lot of them cost more 2nd hand than they do retail (a Battle Damage Spinosaurus is nearly impossible to find under $200). And if a good lot shows up somewhere for sale, a collector will snatch it up quick. I’m a member of multiple Jurassic toy collecting groups, folks hunt those sites for rare items, good bargains, etc.

Best of luck. And I hope your kid has the best birthday ever. I LOVE that my son is into dinosaurs (and all prehistoric creatures) so much. It’s great.