It's tough that you already put down a cage floor, because it's hard to put in belly heat then. You could also have routered a channel into the floor and put heat cable in the channel, and then covered that with a thin sheet of rigid waterproof plastic (like shower surround material).
I use heat cable as overhead heat in my green tree python's cage and it works well, but she doesn't need belly heat.
Again, if you don't want to rip up the floor you just put in, your best bet would be a radiant heat panel. You also could possibly put flexwatt or other heating pad on the wall of the cage, though it wouldn't look pretty. If you don't mind re-covering your floor, you can put flexwatt over your peel/stick tiles and then, as the other person mentioned, put something over it. I wouldn't recommend glass because heat differences might cause it to crack, but you could use thin acrylic/plexi or shower surround plastic. You could cover the whole floor with it and silicone the edges as well.
Might want to ask in the caging forum (look in the General forums) for more help! 
-----
Lauren Madar - OphidiaGems.com
1.0 Ball Python, 1.1 Hog Island Boas, 1.1 Hypo BCI, 1.1 Surinam BCC, 0.1 Green Tree Python