If you catch one on ebay, it will be a lucky find and really pricey. From what I understand DDay doesn't even allow shoulder-fired nerfs so a piece of PVC painted and modded to look like an AT-4 with a marker inside would work.
Kent Island Sea Monkeys - Semper Gumby
PAINTBALL ADVENTURE PARK
Just mock up an old marker as a large caliber AT rifle. Make it look like a Barrett light .50, or period appropriate Boys AT rifle or whatever. Add a piece of 1" PVC over the markers barrel to extend the look of the barrel, but add a rail underneath and on the sides to keep it from bending under its own weight.