Seachem NPK? Meaning Excel?
You can setup a DIY CO2 setup fairly easily with many things you have around the home.
1) 2L bottle
2) Yeast (champagne yeast ordered online would be best)
3) Airline tubing from any of your existing aquarium setups
4) Drill/drill bit. A hot nail may or may not work.
5) Check valve. I can mail you one for a few bucks if you want, Rex Grigg also sells them, you can use the plastic ones too, but brass/metal is always better.
6) An additional bottle if you want a gas separator/bubble counter
The bottles you may probably have at home, same with the airline tubing. You'll only need to find the yeast and the metal check valve online.
Instead of the bulkhead fitting, use a drill bit if you can and drill a hole in the cap of the 2L bottle. Make sure the hole is slightly smaller than the outer diameter of your airline tubing. Once you do that, you can cut one of the airline tubing so that it has a point/angle to it. Thread it through the top of the cap and pull with a pair of pliers or something. That should give you a good seal by itself.