Hi Matt and welcome to the site.
If you have not yet done so, I would go the the Home page and in the Airport/ARTCC Code box type in KPHL. This
will bring up Phila international and you will see listed approach/departure freqs for the airport. Give a listen and
you will hear departure handing off the aircraft to ARTCC centers. This will give you current frequencies in use for
You can also type in the same box either ZNY or ZDC and get what frequencies for New York and Washington Centers
are covered by Live ATC. You will find some that handle traffic to and from Phila.
If you have a scanner you can go to the Radio Reference web site. They have a good, up-to-date listing of all
frequencies for centers in the USA. Again check NY and DC for frequencies. This will help also.