Pirate101 prepaid cards are definitely much harder to find than Wizard101 ones. In fact, I get super excited whenever I see them in person!
My Toys R Us usually carries them, but I haven't found them anywhere else. I did get really lucky one time and found an Empire Bundle at a GameStop awhile back. I tried purchasing it, but the cashier couldn't ring it up. They ended up giving me an e-code for it and took the rest of the P101 cards off the display.
Once a super fan, now a KingsIsle QA Tester! I squash bugs.
You can use Wizard101 prepaid cards equally in both games. In my area, Family Dollar & Dollar General have both stopped carrying these game cards ( as well as few others ) people were scamming with these cards, returning them to the store to get a refund of cash. My local CVS now requires you to sign a waiver, if the card doesn't work, you must contact KI to work it out; CVS will no longer refund card purchases.