Where can I buy silicone ear tips for my AirPods Pro?

There is also a huge array of eartips available places like Amazon. I bought replacement ones there because the Apple ones just won't stay in my ears.

You can buy them at apple or at an authorized reseller

See this link for the apple store with the eartips you need

https://www.apple.com/shop/product/MY3V2ZM/A/oortips-voor-airpods-pro-1e-generatie-2-paar-medium?afid=p238%7Csvbr93AdC-dc_mtid_187079nc38483_pcrid_652687720008_pgrid_150063812080_pntwk_g_pchan_local_pexid__&cid=aos-nl-kwgo-pla-btb_lia--slid---product-MY3V2ZM/A-NL

AirPods Pro (2nd generation) Ear Tips - 2 sets (Large) - Apple

this is for the airpods pro 2nd gen