Even if you did find it, you will need the 609 license to purchase it.
There are several retailers that sell it regularly on ebay(I have bought some, don't remember the name). You can buy 2 12oz cans and the tap for about 69.00
More importantly, you should find out why the sytem is low as it has a leak somewhere and you would just throw your money away. Never mind the legalities.
Do a search and you will find lots of verbal warfare regarding refrigerants.
:p