Thanks for all of this helpful information, everyone. My buddy is picking up the grill tonight, though he might not be shelling out the cash to fix up the gas assist right away. He reminded me that the grill will work perfectly fine without the gas assist, and he's certainly right about that.
So I did some calling around today, and found that the RV dealerships I was hopeful could help were mostly without 5# tanks. One had one in stock, but it was like 120 bucks (no thanks). None of the hardware stores I called had any, but I'm led to believe that my local Lowes can get one for me for 53 bucks. So I might go that route. And as AJ points out, the mighty Amazon can deliver one to my door.
I suppose there's a way to buy some connections to enable the use of those disposable 1lb tanks like the modern Performers use... I should look into that at the hardware store.
Man, I love those SS Performers... someday I'm gonna find me a red one.