I have had my best bargains from local timber merchants that have a sideline in surplus stocks and ends-or-ranges from major manufacturers and suppliers, and one from a remote Magnet warehouse clearing remainder stock of the exact range I wanted (I had to buy the entire stock, by pallet loads)
However, Benjamin James seems quite a good supplier, they are useful because they will make cabinets to order in a wide vaiety of good quality colours and finishes. I have not bought doors from them though. Due to the delivery charge, they are expensive for small orders.
I've bought shelves and panels made to order from Trade Kitchens EU and quality has been good.
If you have a van and can collect, you can get fantastic bargains from prosperous homeowners throwing out a perfectly good kitchen because their whim has changed. This is one I recently watched
ebay
and also
ebay again
Most people haven't got a van so bids are few, prices low, and the vendor wants to avoid the effort of taking the stuff to the tip.
If buying second-hand, buy plenty because there will be some items that are damaged or don't fit.
I have used ebay suppliers when trying to match disconyinued doors, but they are usually small traders who've had the luck to buy at a warehouse clearance.
This sort of place sells new doors, but they are mostly vinyl wrapped MDF. Vinyl peels off from heat, steam or damp, so is unsuitable for a room that contains taps, kettles, sinks, dishwasher, ovens or pets drinking bowls.
on ebay
I can't vouch for the quality of these companies, but they are examples.