Amiibos are a bit expensive. There are a few DK amiibos, they should all work with Smash because amiibos have a character identifier that’s recognized between games and amiibo series.

The ones from smash bros and super mario series should be around the “normal” price (for me it’s about 13-15 euros). Maybe you can find a second hand one for cheaper.

There’s a new one with DK and Pauline for Bananza, like other new amiibos it costs a bit more (around 18€ for me). It unlocks a costume in Bananza as an extra.

There’s also an old Skylander DK figure that doubles as an amiibo if I remember correctly. It’s kinda ugly and cheap-looking, and probably not easy to find, but who knows.