About ten years ago I was in the importing business in the Caribbean and imported many aluminum boat trailer kits, for 13 foot skiffs to 28 foot sailboats. I got them from a company called American Trailers. They had every part to build the trailer and came on a couple of long pallets. Great trailers, cost effective, especially being shipped dis-assembled to the Virgin Islands. I have searched over and over for them, or a similar supplier, without success.
About the only similar thing I see now is a very lame group of trailers from Trailex. They are not cheap, either. If I needed an aluminum trailer, I would just buy aluminum channel, and a trailer kit from Southwest Wheel. They sell kits with everything to build a trailer, except the actual frame. Some have torsion axles, even. You can make all the joints with corner angles and stainless steel bolts. I would probably use a galvanized square tube for the tongue, or a very heavy wall aluminum square tube. Paint liberally anything that's not galvanized or aluninum. Also, trailerpartsdepot.com will sell you almost anything to build a trailer, very reasonably.
Good luck! IslandJim
I'm an Eighth Day Adventist. On the Eighth Day, God went fishing!