You can buy Universal gift cards at local grocery stores and drug stores in Orlando. I know some Publix stores have them and then either CVS or Walgreens. They're sold in denominations of $25, $50 etc and you're left with what the store has left.
You cannot purchase these at Universal or on their website. Furthermore they are not redeemable once inside the park, you have to turn it in and get universal dollars for them and then use that money.
I personally think all this is a big waste of time! Until they get a gift card system that is a little more user friendly, it seems more hassle than it's worth.