I'm sure there are a lot of stores here in Edmonton that sell graphic t-shirts. Some that I can think of at the top of my head would be Warehouse One, Stitches, Garage, places like that. I'd say that Warehouse One is likely your best option, as their clothing is relatively inexpensive and they often have sales of some sort. Although I generally don't personally like their tops, it's the best store I can think of. I'd recommend that you perhaps go to the South Edmonton Common, West Edmonton Mall and Southgate Mall websites to see their directories and to then look up the individual stores to see what kind of clothing they sell and their price ranges. Another store I just thought of, Bluenotes/Thrifty's. Another cheap store that does sell graphic t's.
PacSun has cute surfer and skater clothes...and for reasonable prices too.
ciao for now,