"topping off" a "virgin" ac system

[joesquid]

I have an original compressor/evap system from a Vendo 144 that's no longer cooling.  The compressor and both fans run but the ac line frosts up only to the first pass of the evaporater.  I'm assuming it needs a boost but this system is still originally sealed, i.e. no valves.  How do you go about adding freon to the machine?  I noticed another post where "Freeze 12" was noted as acceptable for boosting an R-12 system.  Where can you get it?  I'd like to find a way of doing this myself (in order to reduce costs).  Is this possible or should I go find someone to do it?