The phrase "grab a snack" is correct and usable in written English. You could use this phrase when suggesting that someone take a break to eat a small snack. For example, you might say "Let's take a break and grab a snack before we finish up this project.".
It's grab a snack here or there.
I never just grab a snack for lunch.
This would be a good time to grab a snack.
Don't forget to grab a snack at QUIZNOS while you're in that corner.
That's when Tom Hanks will come in and grab a snack".
Like other press-box folks, I often use the interval to grab a snack or visit the rest room.
Visit it, look around, join the chattering, jostling crowd, listen to the noodle vendor's spiel, grab a snack.
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK