I agree with Lisa - it is a plug-in (small program) that you download - it shows up on the menu bar of your internet browser as the PayPal "P" with a drop-down menu. Whenever you're on a site that doesn't accept PayPal directly, you go to your plug-in and say "generate secure card". It will randomly generate a one-time use credit card number that is linked to your PayPal account - to the merchant you are just using a Mastercard but the money comes from PayPal.
I have been using it for several months and I *LOVE* it - it has never failed me and they are pretty good with security too - I've gotten a couple of verification phone calls to make sure I was the one who authorized the purchase. Another upside is that they are truly one-time use cards, so if security is somehow compromised, the thieves won't get too far with it (if anywhere).
Another possible option would be to use a PayPal debit card - you can request one from your account. This wouldn't work if you need it right away though - it can take a few weeks to get it in the mail.
ETA: I guess I didn't point you to any sites - I have used both Maple Leaf Tickets and Undercover Tourist with good results!