Just stumbled across your post, I had the same problem as you 18 months ago when I was trying to get my MC21 going.
I managed to buy some solenoids on ebay but struggled to get the x piece. There were some for ridiculous money, so I made my own:
One leg of the X has a jet inside, from memory I think it is 0.65 mm but you need to double check. I already had a blank aluminum jet which I carefully drilled with a 0.6mm drill bit, I used a Dremel and a small drill chuck to hold the jet.
I got a plastic cross and tapped a thread into one of its legs, then screwed the home made jet into that leg.
I didn't need to make a Y-piece.

The X and Y pieces I purchased from Advanced Fluid Solutions:
NOR-KS-04 4mm nylon 4=way joiner (cross)
NOR-YS-04 4mm 3 way joiner (Y-piece)

Rubber hose from ebay. I got several different sizes, suitable for fuel, even though only air goes through these pipes.

And some 0.6mm, 0.7 and 0.8 drills from UKDrills.com (several of each in case of breakages)

I might have enough bits to make another X-piece.

Good luck
Regards, Ian