Modern computers have built in support for Thai, so you don't need to download a Thai font in order to type in Thai on your computer. The default Thai font is clear and easy to read, but obviously it's not the most interesting looking font in the world.
If you're looking for more stylistic fonts, you'll need to download them. By far the best place for this is f0nt.com, a website where independent Thai font designers have uploaded some high quality fonts and made them freely available. The website is only in Thai, but it's easy enough to find the download link on each page even so - look for the large "ดาวน์โหลด!" button.
Some recommended fonts with links to download them are:
|กรุงเทพมหานคร||( default font, for comparsion )|
Note that in order to be able to use these fonts, you'll also have to set your computer up to be able to type in Thai too.