Unfortunately, what I found were a lot of locked doors, and buildings that had been converted for new uses (or demolished altogether). Here are the names and addresses I researched, and what you will find if you go there.
Stop #1: Xtremez Paintball, Tualatin
Last stop: Oregon Paintball
Address: 40 NW 2nd Street; Gresham, OR
By now I was thinking that Paintball was dead and gone from the Portland area, but I decided to give this one last shop a look. For a moment, I felt like the M&M's in that Christmas commercial where they come out into the living room and see Santa and the red one says "He does exist!" And then Santa says, "They do exist!" and then they both feint and the Yellow M&M is left standing there looking all kinds of confused.
Oregon Paintball is alive and well. Unfortunately for me, they close on Sundays at 3pm (They are open Monday - Saturday 11-6) and I didn't arrive until 3:40. But peering in the windows, the shop looks fully stocked and ready for action. I plan on heading back out to see these guys on a day when I'm not wasting time driving all across town.