It's not a pretty site, but most of the types you're likely to find at the dealers will have good links. Identifying local dealers is a little "iffy." The RV dealers seem to swap brands frequently.
For used trailers, you might try RV Classified dot com. Lots of listings, but note that a lot of dealers put blind ads up there. Ignore anything that doesn't show an asking price. Photos of quite a few used things.
Used trailers of almost any description sell quickly in my area, unless they're not realistically priced. If you decide what you want (or what your friend wants) be prepared to GO if you identify a good one. And be suspicious of anything that's on the market for more than a week.
New "hardwall" travel trailers start at around $13K - $15K in the smallest sizes - plus or minus a few thou. Pop-ups are a little cheaper, but less secure. There is no upper limit on how much you can spend.
As to used prices, we sold our 7 year old 12-foot Jayco popup for $3K within 2 hours after the ad hit the street in the newsrag. That was discounted about $1K below "book" because of a dead refrigerator (quoted replacement, $980).
Most of the pop-ups can be towed with about any vehicle you have. Most travel trailers (including "pop-outs") will force you to exceed manufacturer's ratings for your tow vehicle. The majority of people who are "into" travel trailers end up, soon, getting a "dedicated" tow vehicle. Two of our friends have "burned out" a total of 3 heavy duty (1-ton or better) trucks in the past few years towing relatively modest sized trailers back and forth to Winfield and surrounding areas. (No comment on how "sensibly" I believe they were travelling.)