CallingMart carries PINs for most prepaid operators at a discount of up to 5%. Plus if you use coupon code ca3p-1207 you get will get an extra 3% off on all refill orders over $18. This coupon is good indefinitely.
I've been buying all my refill PINs at CallingMart.com for several years now. I've found them very reliable and PINs are delivered instantly (the PIN number appears on the screen as soon as you checkout). The first time you order from CallingMart they will call you back within an hour to verify you identity. After the first order delivery is instant.