Go to a full service propane service place.
Bill's propane in Phoenix does my work and certification 480-610-6751 for my Squarestream horizotal bottles.He has always done me right.I am sure there must be one in Tucson.
Also look under Squarestream on the trailer forums of the Airforums at "Squarestream propane bottles"
And look at "1988 Squarestream 29 ft project".Leefield replaced his with steel bottles.You could PM him if you need more info about where he got his.
Remember the horizontal Aluminum bottles for a TRAILER are hard to find.I have been told that the ones used on like forklifts will not work on trailer.
DO NOT JUNK YOUR ALUMINUM BOTTLES.If your going to junk them PM me first.