I just went through an absolute nightmare trying to find panels to fit my arch. I wanted to get the most
power possible, but had it in the morning rather rigid maximum size constraints. Amazon is probably the best because they provide dimensions. They will also sell you small quantities, as in one or two panels. There are companies that specialize in
solar panels, but they will rarely sell less than 5 or 10 panels at a time. They also very the dimensions in the downloadable in spec
sheets rather than listed right on the product description.
As you look at
solar panel vendors and offerings, remember that you will not get more than about 200 watts per square meter. That's an easy to remember number to do an easy reality check on a panel that seems too good to be true. It's not as easy to remember, and I don't, but it is also worth doing a check on watts per pound. Pull up one you know, such as renogy, and calculate watts per pound as a comparison against other vendors. If their listed weight is 60% of what it ought to be, that would be a red flag.
Someone cautioned on flexible panels. My
boat came with some Solbian (a premium vendor) flexible panels about a decade old. The surface is a hazy gray and the output is about 30% of rated. Flexible is nice, and they do weigh less, but I don't think they have longevity and they certainly cost more.