I had the same problems when getting quotes in for my 7" scale Fowler lorry. AJB boilers were about to retire, and I suspect have subsequently done so. The players that I know of are:
Berwyn Steam Fabrications – 01691860570 – currently advertising in Narrow Gauge World for 4" TE boilers up to fullsize.
Station Road Steam – bit selective on what they take on, but worth a call
TRS Boiler Makers, Derbyshire 01332 865616 – Excellent quality, but a long waiting list.
H A Mc Ewen – Can do them but only gold plated judging by the price.
Bennett Boilers 01278 792255 – I used these and was very impressed with quality of build. More used to full size, so they may not want to take on 4" scale work.
Statfold Engineering – have heard rumours they do TE boilers, but no personal experience.
North Bay Engineering – have heard rumours they do TE boilers, but no personal experience.
Andrew Kirk at Holt Boilers 01964 529392 – I have seen their work and would avoid them. Very good price, though.
I did not find that going to an industrial pressure vessel fabricators would get a good price. As Jelly describes, I had detailed drawings, everything signed off by BES. I tried one of our North Sea suppliers up hear in NE Scotland. They wanted 4 times what I paid for the boiler. Of course, you might strike lucky, but if you are building from something like the Plastow drawings, an industrial fabricator is likely to run a mile. Those that specialise in the game, have their own version of each model boiler that they have agreed and signed off with their inspection house.
I hope somebody else chips in with a few other names,
Martin