I am entering Belize from it western border with Guatemala. I have USA multiple entry visa, which I know means I get I can get a Belize visa on arrival (with 50 USD fee). However, is that possible from the border as well?
It is entirely possible to get a visa on arrival at the border, driving / crossing in from Guatemala.
However, note that for many countries, no visa is required for entrance. You can get a 90 day stay stamp basically without question.
For examples, stories, and pictures of the whole process, including some problems with border hopping, some bloggers have posted their experiences online, reenforcing that you can indeed get a visa on arrival. Note, that it does depend on your country though, and you'll want to check the list above for yours.