Kitchen cabinets, matching doors.

We are considering buying kitchen cabinets from local suppliers, placing them and cutting/fitting a work surface to the top. We would like to match the doors to the existing cabinets we have, but don't know the supplier.
Can anybody suggest why this is not an easy job?

Yeah, you may well be correct - but that is giving up before I start.
It may be a market for a web site which would match existing fittings to manufacturers specifications to find out previous suppliers. It could apply to a wide variety of products - not just construction, where there is a common requirement to match the existing products.