My understanding of TSA rules is that you are allowed to take knitting needles on domestic flights. You might consult their Web site or call an airport just to make sure, though. Often people have the rule about allowing knitting needles on planes printed out just in case some uninformed worker gives them flak about it.
A quick search online turned up Knit One in Pittsburgh, PA (if that's the Pittsburgh you're talking about): http://www.knitone.biz/. It looks like a neat store, but I also found a few others just by searching for the name of the city and "yarn." You also might look through the online yellow pages.
I hope you have a wonderful, fun, yarn-filled trip! Oh, and enjoy visiting your brother, too.