"Just where to buy one?"
Since Omega has stopped selling all but a few parts to 'unauthorized' repair shops and parts supply houses, movements are very hard to find and high $$. For this reason, I would consider a regular ETA 255.xxx as long as it fits Ok in the case. The hands will fit but I do not remember if there were any issues with the new movement being too thin or anything else.
Since the watch is 20+ years old and not a collector item the different movement will not hurt the value very much.
As far as that goes, an ETA 955.xxx might also work and they are a lot less $$. Just be sure to get one the same size and date offset as the original movement because 955.xxx movements come in 10 1/2L and 11 1/2 L sizes. The ETA/Omega 1438 is 2.5mm thick same as an ETA 955.xxx and a regular ETA 255.xxx shows to be 2.45mm thick. Not enough difference to matter.
There are quite a few parts supply houses listed on the internet that will have new ETA movements...Cousins in the UK for instance.
"Even paid £350 (?) to have it 'serviced' they then quoted £1000+ to repair the watch I bought new for $1000."
They sound like a gang of crooks. I've 'serviced' quite a few ETA 255. and 955. movements and the only hard thing about it is lining the step motor pivots up in the plates because of magnetic attraction.
Many/most repair shops today have so much overhead that they have to 'steal' $$ from customers to survive. That is their problem, not yours.
Imho the motto of most of today's watch repair shops should be 'Rob 'em and Rape 'em with a Smile.' Ha!
What you need is someone who can do the work at a reasonable price and does not try to make a month's pay out of a two or three hour job.
Other than finding an original Omega movement, the movement swap is very easy. Maybe ask friends, family etc. if they know a retired watch repair guy who still does repair work at home.