There isn't a lot of veggie food here, at Travellers hostel there's a bar and restaurant down stairs that do 2 decent vegetarian meals both were 89 CZK each.
There's a restaurant called Liabon which is 100% veggie but really expensive.
There's a wicked kebab shop that's sort of out of the way but do a mean falafel kebab, ask anyone they will point you in the right direction.
I suggest making your way down to tesco, it's a bit of a walk but doable! There are a few mini marts in the town but they are a lot more expensive and don't have much.