Well howdens and benchmarx are really good for kitchens and also have various sizes of doors to match your carcasses. You will have to go with a tradesman as it is trade based. It all depends if you want a new kitchen layout and on your unit sizes. If you want a new layout it would be better to see what units you have got and whether you can position them out to fit new layout if not a new kitchen but get a free plan and see if you can use any of the old units so that you can keep cost down.
Hope this helps
Andy

Hi there are several companies on the web who offer replacement doors, just google it, i wouldnt recommend using Howdens or Benchmarx for this, the quality of their doors isnt great & they come pre drilled for their hinges which can vary slightly in position from different carcase manufacturers. You are better off to either measure your hinge positions and most replacement door companies will pre drill your doors to your dimensions, but this can be tricky, when we replace doors we will drill them onsite to ensure a good fit, also it may be worth you investing in newer hinges & have these replaced too (unless the ones you have already are perfectly good). Hope this helps