I bought new kitchen doors of my choice. In standard sizes they usually fit straight into the existing hinge holes.
depending what you want, it starts at about £20-£30 per door. Solid hardwood doors are more expensive.
In my case I bought the remaining stock of a discontinued line from a Magnet warehouse, including plinths and pillars, I think it was about £400 and I had plenty left over, but there were no pan-drawer fronts, which I eventually bought second-hand.
You can also get end-of-range doors and drawer fronts on ebay, usually new in the packaging, but in this case it is essential to check they have all the sizes you need, as it may be difficult to find matching ones.
I'd advise avoiding vinyl-wrap doors, because it peels off if it sees steam or heat, or if a cat scrabbles at it.
I've had a few things from Benjamin James and been very pleased with the quality, but only laminated slab doors, which are very durable.
Most budget doors are moulded MDF with a sprayed paint or a vinyl wrap finish.
If you are budget-conscious and handy, with a hired van, you can pick up used kitchens very cheap when people refit their kitchens (some do it every three years). You must inspect them for condition and quality before bidding, and it needs to be fairly local. Most buyers haven't got a van, and the vendors are stuck with taking the old stuff to the tip. You will find them on ebay. For example I wanted two used pan drawers to match my new kitchen, and ended up buying all this (see pic) plus all the units from the other side of the vendors kitchen, for £330. I also paid a chippy for a day to help dismantle and load, and there was the cost of a large van, so double that for total cost. I had been looking for a long time, and think I was rather lucky. It could have been in Aberdeen or somewhere, and not a realistic proposition for me.