The pot is probably available(if you can figure out the value and taper) from Digikey. Having said that, you might also be able to get the part from an Avid-approved service shop(like allproelectronics.com based in Nashville, TN).
Another option is to not use the knob on the 003 and add a monitor/source selector to your setup. I've been using a Presonus Central Station for years and can't imagine not having all that functionality. The Central Station does have a small issue(design flaw?) in that whenever you turn the volume down to the bottom 20%, the image shifts off the the right(verified on at least 5 of them in 3 studios), but it happens at a volume that nobody would be paying attention to. Tops on my shopping list right now(as a replacement) is the Audient Nero($499).
No doubt you are wondering if another piece of hardware is worth the money, so here's why I can't live without it(BTW, I also am running a 003R)
#1-I can switch between Pro Tools audio(on RCA SPDIF), computer audio(on lightpipe SPDIF), computer 2 audio(second PC for non-audio stuff), iPhone(via mini TRS cable hanging out of the rack) and a second set of outputs from PT(long story, but something I need).
#2-I can switch between 2 sets of speakers, with or without a subwoofer. Food for thought