Usually vets listed as exotic vets treat your type of critter. I'm not in your area so I can't recommend anyone, but I'd open the phone book and try that - or the internet yellow pages. Alternatively, do you have other more standard pets, or friends that do? I'm sure a more traditional vet can tell you vets in your area that treat gerbils, etc.
I found this in a very quick google search...I'm sure you can find more in a more thorough search; I have no idea how far this is from you.
I lived in Wynnewood (Montgomery County, on the R5 train line). Are you in Philadelphia itself or in the suburbs? If you live anywhere near where I did, I took all of my small animals to Dr. Royster on Lancaster Ave (route 30) in Haverford, just down the street from the haverford train station. He treated all of my hamsters and rats. He removed a tumor from one of my hamsters and removed a benign growth from one of my rats.
Not sure if Ardmore is too far away from where you are, but I thought I'd mention it.
I hope you already found help but if not Radnor Vet in Wayne is wonderful!
I LOVE Dr Case-Miller and Dr Donato. I bring my buns there all the time. Dr Case Miller just did Penelopes spay and saw her the next day too for pain meds.
I agree - Dr. Donato from Radnor vet is great. He cared for one of my degus and helped bring her back from a very bad respiratory infection. Conchester Animal Hospital in Boothwyn will also work on small animals. They don't seem as knowledgeable, but they'd be good for a check up or emergency.