Sourcing apropriate plastic spacers/seperators/stand-offs for A4000 accelerator.

[Castellen]

You could try the likes of Farnell or RS, they have lots of different PCB mounting spacers.
A good alternative for you would be to use the threaded spacers.
They are basically a short tube with a threaded hole in each end.  You need to put one screw (with a locking washer) in from the underside of the motherboard, and you screw the CPU board in using an extra screw from the top.
The spacer length is 17.5mm.

[Castellen]

You mean the nex-nuts that screw in either side of the D-range connectors, all screwed together to make a pillar?
That's a pretty neat idea, never thought of that.
Just need a nut on the underside of the motherboard to hold each pillar in place.

Some LocTite (or other thread glue) between each of the hex nuts would stop them unthreading when you remove the top screw holding in the CPU board.