When you search the market for camper vans, you will mostly find Class B motorhomes built to a more luxurious standard, from RoadTrek, PleasureWay, Great West, Airstream, and more recently Winnebago. Some of these will be built as two person homes, others as family travel vehicles with kitchen and bathroom facilities.
For something closer to the camper van side of the spectrum, look for the Pleasureway Traverse, which was built on Ford's E-150 van, until the E-series was discontinued. This had a small kitchen area and a rear seat that folded into a bed, with a second bed in the pop-top, on the pattern of the campers that were build on the VW Type 2 van for many years by Westfalia in Europe and Sportmobile in the U.S.
Most camper-like of current Class B builds by major manufacturers would be Winnebago's Travato, on Ram Promaster 20-foot high-top van. But that one is only slightly less expensive than a Class B built on Sprinter or Transit, and it hasn't been around long enough to be a bargain used.
Or you could look for a VW camper van. First generation of the Type 2 has become a collector car (especially the camper) and will be expensive if fully restored. Market for the second generation is still enthusiast but moving towards collector prices. Most popular today is third generation, called Vanagon (or T3) which has not become collectable in a big way and is more competent on today's highways than earlier Transporters, and sell for $30,000-40,000 in usable condition. Most recent VW Camper was a Westfalia build on the T4 van, sold in this country as the Winnebago Camper Van, 1999 through 2003, prices about the same as the Vanagon.
Next up, Daimler-Benz bought Westfalia, and had them build a Westfalia camper on the Sprinter platform. This was a lot more like a luxury B motorhome with some overhead sleeping space than a basic camper. This one was sold in the U.S. as the Airstream Westfalia, briefly. This are rare, in high demand, priced accordingly, i.e. you might find one 12 years old for $50-60,000, not that much less than what it cost new.
Sportmobile is the biggest builder of custom-built B motorhomes and camper vans, what you find in the used market will depend on what each original buyer wanted. They will build on a van you bring to them, but they discourage this as false economy, on the premise that putting a lot of money into a (usually high mileage) used commercial van will get you a camper that you will not use for very long. That of course depends on how you use it, how many more miles are in your RV plan.
There are many smaller builders of camping vans, as well as builders of fittings to build into or even slip into full size vans. When shopping for used vans I often come across examples converted into mobile offices, equipment labs, and the occasional camper. These will often be no brand name on the conversion.
If you would DIY, consider that $20K can get you an Chevy Express 3500 that is 3-5 years old, 40,000 to 80,000 miles. Drop to $15K and you are looking at 150,000 miles typically, same age range. Under $10K and you will find 150,000 to 200,000 miles, but 7 to 16 years old. Commercial vans particularly, if purchased, will be run until the first owner considers them worn out, whether that takes three years or 15 years. If leased, they often come off lease 3-5 years and leases will have 50,000 to 80,000 mile mileage caps. If they were originally well cared for, you can expect 200,000 miles from a late model Express or an E-Series Ford.