I use Neutra Maxx 5000 Gel.
It is 1.1% Sodium Fluoride and comes in a 4.3oz bottle.
I purchase it at the CCC's dental group office for $12.
It says RX only, but I just walk in and buy it.
It seem to work fine in the trays.

I used to use Omni Gel 0.4% Stannous Fluoride.
It stained my teeth, but otherwise worked fine.
They told me that Stannous Fluoride is a bit better than Sodium Fluoride but the difference is insignificant so they switch when staining is a problem.