Mary,
Garlic paste is generally sold as an Italian ingredient, in a tube:
http://www.thekitchenstore.com/amgapa.htmlThe stuff in the jar is generally preserved with some vinegar or brine, which changes the flavor of the garlic substantially (and not for the better). I don't recommend it.
If you don't have any around, just mash a small garlic clove with your garlic smasher. For slightly better flavor, blanch or microwave a large garlic clove or two to soften its flavor and then mash it. Best flavor: use roasted garlic paste.
I've never had canned tomatillos before; I imagine they'd be pretty much like canned tomatoes in how they compare to the fresh product. Best way to find out is to open the can and taste them.