Nice.
I use the ESSTAC set up on a couple of mine too. Added the loop velcro to the outside flap of my chest rig AR mag pouches for an easy way to carry more cards at the ready.
Your best bet for the barrel, tube, and any other parts is to call Mossberg directly and place an order with them.
800-363-3555
Look on their website and find the 20" factory shotgun that has the barrel you want. Write down that model number and ask for a replacement barrel, mag tube, etc. for model "xxxxx" when you call in. It'll save you some time and frustration trying to explain the situation to the CS rep taking the order and it'll be easier for them to look up the part numbers based on an existing shotgun model number. The barrel should run you around $150, $35 for the mag tube, and $8 for the spring based on my last order.
Keep us posted on your progress...