The best and cheapest way is to find a good exhaust shop and have them make it... very simply, they take two 45deg. pieces and cut ovals out of them on the outside radius, then weld the two pices the together.
Mitchell Muffler has them on Ebay for $35 + shipping. I bought a 3" it needed a good cleaning inside were the pipes met from welding slag and a slight mismatch of the holes. The die grinder with a carbide burr cleaned it up.
You can look into a Dr. Gas "X" type, that is what I have on my car, perfect welds and no slag, but most quality muffler shops make their own version probably cheaper than a Dr. Gas unit. I have a bunch of literature from Dr. Gas and they have dyno proven results on increased HP and torque. Also I think all the Winston cup cars use them. In one of the Dr. papers there is a picture of a Winston cup car flipping thru the air showing the underside and you can clearly see the "X" type cross over