4k7 is in the most common 'preferred value' series, hence there being loads of them. Is there not one supplied with the system?

Ignoring the above, There are >300 5K pots listed on Farnell's website! They range from iddy-biddy ones like this to chunky ones like this.

Downside to using Farnell is the £5.95 handling charge for orders below £30, though you could easily make it up to that with sleeving, cable, etc....

Bear in mind the Rapid one linked to above doesn't say whether it's linear or log taper. Would assume linear as it mentioned 'linearity' in the spec, but could also be an odd spec relating to tolerance along the track for a log taper.....

Chris