Hi Colin:some quickie ideas of people set up to do one offs... or who specialize in custom design and building.
1) Sowter
2) Jack Elliano of ElectraPrint
3) dave slagle for chokes and TVC's
4) several of the toroid places for power supply components
and I'm sure there are others. Check with Edcor. Dan Schmalle sez he has had them do custom work for him though I am not sure what quantity levels they might need. Also... Edcor has just intro'd a line of inexpensive PP outputs and maybe some SE guys as well.
just a few ideas.
msl