Use a proper exchange like Coinbase, Kraken, etc they will have reasonably low fees. You can then keep your coins on that exchange or withdraw to your own self-custody wallet.

What i do is buy usdc through coinbase, send it via polygon network to an external wallet, use trocador.app to switch it to whatever i want (note: you need some MATIC to cover gas fees so the first time i would buy MATIC from coinbase as well as the usdc). If coinbase no longer offers matic buy some litecoin and use trocador.app to convert it to matic