Theres a big boat shop in Old Forge, NY called the Mountainman. Someone there last summer had a 25' Old Town on a pretty decent trailer. Wish I could help more but it might be a lead.
I made one by replacing the tongue on a standard (16ft) speedboat trailer with an extra long one (10ft). Seemed to work well, with some extra rollers along its length. Because it was such an ungainly and long trailer, I folded it in half every fall by lifting the tongue and folding it onto the bunks..... until I hurt my shoulder manhandling the extra length of steel. Now the trailer has a home in the underbrush in the off season.
peter
And yes, a pic of it with your Grumman would be fun,
a while back somebody posted a pic of a short vehicle with a very long canoe,
don't remember if it was a war canoe or not but it was long.
But back to the trailers, if you get one of the right design, replacing the tongue is fairly easy and just a bolt in job. (ie, the style that the trailer pivots at the end of the tongue). Here in MN, there are always plenty at this time of the year, in good condition for something in the $300-400 range. Larger and newer and the price goes up.