If you want to use comercial parts (as in regular ledstrips from samsung, bridgelux or get led drivers): if you have a modelnr or partnr of what youre looking for you can try octopart.com . It searches major distributors and retailers for stock and prices. Edit: just seen octopart is already mentioned.
Alibaba: Kingbrite and Meijiu are old legacy suppliers that have been reliable. Kingbrite can make supplement strips (reds, uv etc) and sells some of their boards as diy parts for quite cheapo. I always contact by email to sort out ordering; the app isnt always right in pricing and such.
Bridgelux: ive bought bridgelux locally to prices that was considerably lower than the global distributors (digikey, arrow, mouser). It may be a good idea to check their web where to buy from a local distributor, especially for drivers. They are the most likely part of your diy to fail and if you can get warranty locally it beats sending drivers back to china.
OP: do you have a specific space in mind? The easiest place to start this thing is from space and budget.