Some artists will take in-game money as payment. There are few I've seen right here on RPC that list those sorts of rates instead of (or in addition to) real money rates. If that isn't an option either... Maybe you could buy a gift Visa from a store, then pay online with that instead? Or maybe mail them a gift card (depending on the amount)?
Regardless, your best bet is speaking directly with the artist(s) themselves to learn what they would prefer. Discuss possible alternative options with them. I'm sure you can work something out!
Regardless, your best bet is speaking directly with the artist(s) themselves to learn what they would prefer. Discuss possible alternative options with them. I'm sure you can work something out!