From their help pages:
You can make purchases through Google Wallet using a credit, debit, or
gift card with the following logos:
American Express(USD Only)
Discover (USD Only)
Visa Electron (Outside of US Only)
However, some customers may be able to set up payment through their mobile carrier and people can put money into their Google Wallet with gift cards purchased in store (or online).
My answer above is how users can get money into their Google Play account from 3rd party sources. I guess I misunderstood the question.
If you're selling a game on Google Play, you have to use Google Play for in-app purchases, as stated in the Google Play content policy:
- In-app purchases: Developers offering additional content, services or
functionality within an application downloaded from Google Play must
use Google Play's payment system as the method of payment, except:
- where payment is primarily for physical goods or services (e.g. buying
movie tickets; e.g. buying a publication where the price also includes
a hard copy subscription);
- or where payment is for digital content or
goods that may be consumed outside of the application itself (e.g.
buying songs that can be played on other music players)