Any place to get a durable yet fairly inexpensive liner for a 24ft above ground doughboy swimming pool??

Answer 1

Most liners these days are made from offshore vinyl, so there's no difference in quality of the actual material. You get what you pay for though, there are a few different thicknesses you can get. 27 mil and up and a pattern will cost more than a plain one.
Shop around. Buying online is an option, but you may find a better deal at a pool shop that bought too many of a certain pattern and thickness, that just aren't selling. They'll be wanting to clear them out at season end to make room for the new patterns for next year. I think you'll also get better customer service from a pool shop than an online reseller.