Any recommendations for where to get a headshock service and an idea on cost?

My Lefty is going here shortly as they are local.

Noahs Ark

The official place is here.

Thumprint Tuning

They come highly recommended by those I have met who use them regularly.

Tony at Thumprint is a real star, great service,

Evans do them as does Dave Garland at stendec, full srvice £120-£160 plus postage

Dunno but I know someone who got theirs destroyed a bike shop who didn't know what they were doing and didn't follow the instructions. He ended up putting a broom handle inside it instead of the air cartridge, until he fixed that, at which point he had no damping.
So you're right to be careful where you get it serviced.