Alternatives to Proseal

I have some work that needs me to open up access panels to my fuel tanks. I am having difficulty finding proseal in stock anywhere. Does anyone know an online place where I can purchase it? Any suggestions on other fuel tank sealant products that will do as good of a job?

ACS, skygeek, Van's are all showing proseal and chemseal/flamemaster B2 in stock for me.

As Jacoby states, Aircraft Spruce has it in stock.

Any Polysulfide product that meets requirements of specifications AMS-S-8802 formerly MIL-S-8802, CMNP021, Lockheed, Gulfstream, Hughes Aircraft, Fairchild SPS 6-1 & 6-2 will work.

I have a pneumatic ProSeal caulking gun that I use for fuel tank work. The 2 Oz cartridge works for most small jobs. I use a sealant mixer with the cartridges to make sure they are mixed well before use.