I would think they would work on most sites. They are supposed to be accepted everywhere that credit cards are. The only issue I could foresee is if the company does some type of hold on the card first that ties up the funds on it and then the charge doesn't go through.
You could look into getting a Mastercard debit card for your PayPal account.
https://personal.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=marketing_us/debit_card]
https://personal.paypal.com/us/cgi-b..._us/debit_card[/url]