Man that pisses me off, I just wrote you a long response and went to get a site's address to include and when I returned...blank reply box, well, here it goes again.
If you have a Parker store near you, they sell flexible nylon tubing (same as ARB blue line) and others like polyethylene and polyurethane. I am switching to 1/4" OD polyurethane. I just got some at .32/ft, with fittings to change over at about 2.50 each. www.parker.com/pneumatic
I am using a manual/mechanical pneumatic valve though.
BTW, the fitting on the bulkhead, (called 'center compression') is 3/8-24, the same as 3/16" fittings. This info is straight from ARB, and I confirmed it with the fittings. it is not BSP like commonly believed.
The fittings on the solenoid are 1/8" BSPP, like stated in the ARB directions.
So, you can switch over to 1/4" OD (or something else that's more common than the 5mm OD that the blue line is) by using a 3/16" compression fitting with the compression nut sleeve taken of (this will also have the internal flare to hold the compression sleeve in the bulkhead for the copper tube)and screwing this directly ino the bulkhead fitting. The other end of this one fitting can go to 1/8" NPT or 1/4" NPT etc., then you can go to a compression or push-in type for common OD tubing with another fitting, so you'd be adding one adapter fitting here.
Another place (online even) for tubing is here:
I am not sure if they have a minimum length to order online, but they do list their distributors too.
You say tomato, I say fawk you.