Besides propanels you can also look at armstrong panels - equally good, - but the sizes are a little different and they break down more easily for carrying in a car (I have these and I love them).
http://www.armstrongproducts.com/
Many festivals will require that you have a white tent with white walls - many juried ones will require you send a picture of your set-up with your application. The only problem with the less expensive tents is that they are more apt to blow away or leak water in a rain storm. I would at least make sure that the tent is well anchored with sandbags or some kind of weights no matter what kind you decide to purchase.
A few links of interest
http://www.flourish.com/
http://www.shadeking.com/
http://www.dlrsupply.com/
http://www.newvp.com/pricing.html