The sell them at City Fresh Market, and some of the other Balkan Markets (like Devon) and delis (like Beograd). Of the places mentioned, I would rate Deta's #1, Beograd #2 and City Fresh a distant #3. None of these places are near Wicker Park.
City Fresh Market
3201 W Devon Ave, Chicago, IL
1440 W Devon Ave, Chicago, IL
Beograd Meat Market
2937 W Irving Park Rd, Chicago, IL
The only burek I've tasted in Chicago has come from Deta's. I like what I've had and with the exception of one out of a half-dozen times it's been baked to perfection. A recent sampling was overcooked. What I like most about Deta's is the home-like atmosphere/enviornment. Eating there is like eating in the dining room or kitchen of an old friend or family member. Soups are microwaved (from the fridge), though - and that's been a disappointment for me - though they're good tasting (but not quite "fresh"). Deta's is having a special NYE party/dinner tonight with lots of food and a 3-piece music group and if I didn't already have plans for the night I'd probably head there.