I've used Babblebug:
Babblebug.com | Prepaid wireless refills, prepaid phones, mobile recharges, international topups a couple of times to recharge a Mexican mobile with a US credit card. I found them fast and reliable.
They have refills for many carriers in about 80 countries (for example 19 different carriers/MVNO's in Spain) and seem to also now accept Paypal.
Rob
Sims: Telcel MX, T-Mobile US, Virgin Mobile UK, Orange UK, Knowroaming
Voip: Localphone UK, Anveo US, Google Voice