i found that for me (in the uk), ebay works best. i do tend to end up buying expired film, though, which can be a bit of a gamble, but there's also some relatively cheap non-expired film on there
If youโre cool with B&W, Kentmere is the cheapest 35mm film you can buy. Film prices should be rather uniform online and at physical stores, unless you opt to buy expired film.
If youโre a thriftier person than I am, you can find some gems for much cheaper at a thrift store (the only risk is that they donโt work!). I wanted a replacement for disposable cameras, and found this Canon Sure Shot Owl 35mm camera on Depop. Itโs super fun and easy to use!