As a chompSMS user, you should know there’s a cool thing: you can customize the UI of the app: font, background, pic of contact etc… for pictures, we can do it easily. For fonts, it’s a bit harder, especially for newbie (like me, err).
There’re many ways to add fonts for chompSMS to choose, the easiest way is install the Handcent font pack (5 in total) and go into chompSMS setting page to scan and choose font you want.
Handcent fontpacks can be found (free) here.
If you have more custom fonts, the font you buy for example, you must create an app contains the font, and let chompSMS scan for you, for this process, I found an pretty easy way to do it. Take a look at this guy’s tutorial.
As I could see, the font extension is .ttf only, I haven’t tested with OpenType yet. But it worths a try.