That's where I got mine. Unfortunately they're only available in the 16mm size, but they're good quality and work just fine.C1M pots may be the issue - looks like Tayda have them, but I might be waiting a while.
Believe me, I scoured the globe looking for 24mm ones but couldn't find any. IIRC, I found some but they were either stupid expensive or had a minimum 1000 order.
Every time I've ordered from Tayda, I get my package within 3 days! They do DHL Express for $10US. Also I just checked and they have 577 pieces in stock.
1M Ohm Anti-Log Taper Pot
Since you're using the PTB in a 4 pot configuration, I think I have a schematic (or link) for a version that uses all 500k pots (or maybe 1 x 1M linear pot). If I can find it, I'll send it to you.