Actually, Mark has two different articles on making two different flatbeds. The other one is at:
TheDieselStop.Com - DIY Flatbed
But to double-check on buying one you might install yourself, or having one installed, first realize that a flatbed for a chassis cab will not
fit on a pickup, and vice versa. So if you discuss this with a truck upfitter or bed manufacturer, be sure to specify that you are talking about a pickup and not the more common chassis cab that they usually install a flatbed on.
If you want to do some research first, then look at the sticky at the top of this forum about beds and tow bodies for chassis cab trucks. Most of those upfitters in that post have various types of beds, including flatbeds and stake bodies for pickups. If you notice such things as brand names of truck beds on the road, then you've probably seen Knapheid, Omaha Standard, Reading, Stahl, ABC, and Supreme beds and service bodies. All of those and lots of others are in that sticky post.
Beds and Tow Bodies for Chassis Cab Trucks
Also note that flatbeds and stake bodies are very similar. In fact, you would probably prefer the stake body. The only difference is the "stake" pockets around the outside edge you use to install sideboards and tailgates. A flatbed without at least a short sideboard and tailgate is very frustrating, because stuff falls off the bed too easily. So you'll probably want at least a 2"x4" or 2"x6" sideboard and tailgate to stop that from happening.
In addition to all those sources in that sticky post, your first stop should be to "let your fingers do the walking" and find an upfitter near you. In the yellow pages of the nearest big town or small city, there is proably someone listed under "truck bodies". Even out here in the sticks, we have two such upfitters that carry various name brands of flatbeds and other truck bodies. Their main business is to build oil field work trucks from chassis cabs, but they will also install a flat bed on a pickup frame.
If you DIY, then realize that components are available to finish the job. Headache racks, tool boxes or toolbox/fuel tank combos, side boxes, etc. are available on the internet or from the same truck upfitter you found in the yellow pages, as well as from lots of 4x4 and offroad shops.