Hopefully the garage can fix your flat tire so you don't need to buy a new one. Fixing it is possible if it has a puncture wound, like from a nail, and only costs about $20.
To answer your question, look at the lettering on the tire. It will say something like 225/75/R16 for the tire size. The last number is the rim diameter in inches, and in this example it is a 16 inch rim. Stock Sport KK's had 16's. Limiteds had 17's or 18's, and Jets had even up to 20's I think. If you are not planning on lifting your Jeep, stay with the same tire size. Tire options available will depend on your rims. There are many more out there for the 16's and 17's than the 18's or 20's. TireRack.com is a good place to browse.
2008 Liberty Sport 4x4 Automatic