Prepare to (1) be flexible on size- a little oversize just requires cutting the fiberglass, and (2) redefine affordable- RV windows
are just plain expensive.
Definitely scour RV salvage yards, local and/or online. Shipping is costly, something to consider when shopping online. Check nearby Craigslist.
Worst case, you may have to cough up for new windows or tackle fiberglass patch work to make the opening smaller.
Iím afraid itís one of those things people expect to be simple when they take on a project, but itís not.