Indeed they seem not to be too fussy
The standard issue salamander with the red stripe(s) down the back are favourited of my snakie. Also, here in Nova Scotia (and surely other places) we have these Yello-polka-dotted newts' which snakie also likes very much.
The newts are apparently very good to see around if you are a gardener, as they eat slugs and other plant predators, but they are also good snake treats. They are like salamanders in that they secrete mildly poisonous secretions out of their skin when they are attacked, but garter snakes are immune to this secretion. The newts are more rare than salamanders, but I have found a half dozen or so of them.
Currently I have about 2 dozen normal salamanders frozen in a container in the freezer, I froze a half-inch of water in the bottom of the container first, to attampt to combat freezer-burn, but either way, snakie has no qualms about eating thawed salamanders. I proclaim frozen salamanders a success
Toads and small frogs are also welcomed by the snakes, but the thing about this is the potential for high parasite load in these reptiles. I did give my snakie one frog a number of weeks/months ago, and she loved it, but I have since avoided them since learning of the parasite potential.