I was looking into the process these guys use to print their boards and I think the easiest way would be to get some paper, print your target (on multiple sheets for large ones, like a banner) and use spray adhesive to attach them to whatever size board you have laying around. I looked at 5-6 different methods for printing on cardboard, and they all involve a lot of work or up front cost for supplies. If I was gonna start printing them large scale I'd probably make the investment, but until that day comes I'm gonna stick with this method or just buying the VersaTarget backers.
On a side note, if you're wanting to print your own targets or use a custom image you have as a target check out an application called TargetExpress. They let you test it for like 14 days I believe, then its just 15 bucks to buy it if you like it. You can adjust target size for different ranges, have it print images on multiple sheets (like stated above) and a few other cool features. Hell, for 15 bucks it's worth it to me not having to buy packs of paper that are printed with generic circle targets. Plus I do graphic design stuff on the side so I'm sure I could come up with some interesting custom targets. May have to design a few up for the site here and see what you guys think. Now to just dig my laser printer out of storage.