I saw another post on this and someone said iboat had it in rolls, I found the post and looked at what iboat had it is 24" x 60', all well and good but I have a 1983 Sun Tracker 24 and the aluminum sheet is 25 1/4". Anybody have any other ideas? This is a project boat I am putting together.
You're best and least expensive bet will be to talk to your local metal supplier(s). Much of the aluminum sheet and plate starts off in rolls and is flattened by the either the supplier or someone else up the food chain. For durability in a water based environment you want to use something in the 5000 series of alloy's. You can use heat temepered 6000 series which is easier to form with heat but you sacrafacice some of the anti-corrosion characteristics of the 5000 series. Unfortunately, at 25 1/2" you're looking at buying a full sheet and wasting nearly half of it. The longest full sheets that I know of come out of Washington state and that supplier stocks it in up to 25' lengths. There are also a couple of east coast vendors that you'll find on line that can flatten longer sheets but it would be a custom order.
If you're in the business of building boats and toons you'd be ordering from one of these places regularly and the cost would be figured in with the price of the vessel. In your case it's not going to be cheap so you might look around and may consider an alternative such as modifying it to 24".
I wonder if the 4 x 8 plastic sheets made for bathroom walls would work. It would be easy to cut and easy to find at any home supply store. I think it only comes in white.
Those are not made for exterior use and the UV rays will eat them up in time. Also I don't know how well they would hold up on an unreinforced area as thin as they are, whereas in a bathroom you can fasten it to a solid wall to secure it.