http://www.heatngrow.com.au/products.htm

I think this could be a better investment check your garden suppliers.

Keith

If your windowsill gets direct sun, then it may get warmer than you realize. In fact, it may get too hot too fast once spring/summer hits.

Depending upon what the light there is like (and don't "eye it" and assume "it looks bright to me" -- your eyes are far more sensitive to light at levels lower than what plants require for adequate photosynthesis). If you light levels are insufficient, then you may need to utilize supplemental lighting. Should this be the case, then said lights will almost undoubtedly provide sufficient heat.

But on to heat mats/pads. You may be able to find plant heat mats at nurseries or garden suppliers as Keith indicated. (Don't bother with BBS like Lowes or HD -- they will not carry anything like that.) You might also check petstores or reptile expos. Under the tank heaters (UTH) are very commonly used in the reptile trade. There are even thermostats one can used to help regulate the temps reached.