I was just going to recommend something, until I realised that you aren't in the UK's North East....
Anyway, this end, I don't know Nova's code, but I have bought tubing from my local B+Q (a large UK DIY superstore) quite cheaply. They keep a good range in stock, so here it's just a matter of taking e.g. one of the outlet valves, to see which pipe it fits in. I'm sure you will have an outlet similar your end?
Just took a caliper to my Nova Monochrome. The tubes coming from the processor measure 10mm in diameter. I have some spares and the inside diameter of the rubber tubing is about 8.5mm.
If you mean from the ends of the tap to the container and not from the processor to the tap, the correct size is 6mm internal diameter. I use 3 lengths of 6mm tubing 4 feet long (actually reinforced flexible car brake hose) so I can drain the slots directly into 2 litre containers when I need to clean them out. I keep them separate so I can refill the slots after flushing them out. By using reinforced tubing they are a secure fit onto the taps with no risk of them slipping off when the taps are open.
The closest standard tubing size here to 6mm is 1/4 inch, but I found it to be a pretty tight fit on the tap barbs. The 3/8 inch (9.5mm) tubing I use fits the taps snugly with no leaks.
But, YMMV.
FWIW, usplastic.com does carry some metric sized tubing of various kinds. I think they only sell it in full rolls however. Aside from automotive parts suppliers for OEM parts, metric sized tubing is hard to find here, IME.
Hi, I'm just completing DYI project of 16x20 3 slot slanted B&W non heated processor based on Nova design. To avoid R&D to recreate drain system of Nova I've ordered their spares kit 2 containing acrylic pipes, special glue and rubber pipes with valves. All of the tubing is 10mm diameter.
If I may add: use rubber tubing. Silicone tubing, available locally, didn't work for me. I eventually found an ebay dealer (I'm in Germany) selling the red lab tubing familiar to anyone, who worked in a chemistry lab.