I couldn't tell you where to buy them. Here I had several places to chose from. I guess you could make a few calls.

To cut them in two. I basicaly used a 24" level to get the barrels as true as possible, then marked verticle lines down both sides. I used a little geometry to find the intersection for the true centerline. If I remember it is pi x dia. =circumference. Then divide by 2. This will tell you where to place your marks for the verticle lines. After cutting, use a good jig saw, lay them out and cut off the ends. Leave one end on for the forward most barrel. Once you start to overlap the barrels, you will have to trim down one edge about a 1/4 " to allow it to slide under the barrel forward and aft of it. So basically , you will end up with one ontop,one under,one ontop,one under,etc.etc. until you get your desired length. I overlaped about 5".

I mounted this to 1/2" treated ply that I put treated 1x2 as a rail that runs full length. The barrels were attatched to the wood using sheet metal scews and washers. Once I started, I rivoted the next one to the first. I placed caulk between the overlaps. It won't stick, but I hoped it would at least form a gasket. Once the whole assembly is finished, I went back over the seams with a GOOD COMMERCIAL hot glue gun. Go slowly and allow it to soften the plastic surfaces as it glues. So far it's holding well.