Here is a link to a site that sells small (2.5 gge tanks, 85 lbs) inexpensive Type 1 (steel) tanks: http://www.americancng.com/categories/Cylinder
. A similar Type III tank (only 46 lbs) is found here (no price): http://www.luxfercylinders.com/produ...specifications
You will need strong brackets to mount the tanks without damaging them. They can be expensive. You will need alot more parts to convert to CNG from propane. Here is a schematic for a low-pressure CNG conversion for a carburated automotive application: http://www.pngtechnologies.com/PDF_F..._Schematic.pdf
. You will need most of the same components. Here is a site that sells some CNG/Propane conversion parts for small engines: http://www.propanecarbs.com/tri_fuel_kits.html
. They do not have duel-fuel kits for mower engines. A site for CNG info for autos and some other conversions: http://cngchat.com/forum/forum.php
Beware, I have no practical experience with CNG. I have only done a bit of research. Here are three cautions I have come across: (1) CNG gas stations may not allow you to fuel your mowers, since they are not factory conversions. In my area, there is only one station, so home refueling would be my main option. (2) Natural gas burns hot; in specially built engines they use hardened valves. You may be able to counteract this with a water-injection system. (3) Engines produce less power on natural gas than propane or gasoline.