Well,, you can build your own Nitrogen top off kit.. Yeah, I bought a three gallon portable compressed air tank from Harbor Freight, I use a regular air compressor.
Now, this is very simple, and basic. We all know that the air your breath,,Yes that right,, inhale deeply,,now exhale, that air. Contains 21% oxygen, 78% Nitrogen,and 1% other gasses! (depending on dinner)!
Nitrogen is a thinner gas than Oxygen!
So, if you just turn the portable air tank upside down and bleed off about 20% of the contents, then fill again and do that for about four rotations of fill,bleed fill, you will wind up with a 98% Nitrogen tank,,and thus have a Nitrogen refill kit!!
If, you have a difficult time with this, just PM me and I will sell you a modified air tank with the fill valve on the bottom (I purchased 100 of these from the UK)! Also,, these tanks come in the Nitrogen Green color so they do not get mixed with the regular (USA) tanks!