Building your own SMPS will be much more expensive than buying one. I know, I have a reel of expensive double insulated mains wire (probably $80 plus $20 delivery from the UK) and a big box of coil formers and such from Digikey (expect to pay $10 to $20 per transformer plus the delivery etc), have tried several SMPS chips with and without internal FET (another $10 to $20 per chip as you'll need to get at least 10, you will blow up about 5 of them), the FETs, the voltage references, the opto isolators... etc etc. Why did I do this? Because I was willing to spend thousands in cash and time to learn about SMPSs. If you are too, then do it!
A cheaper route is to pull apart broken SMPSs and put together a Frankenstein with the controller from this, the transformer from that, etc. But, this requires MUCH more skill and frankly if you have that skill you should be working at a high salary not making one off PSUs. Also the transformers and other parts are quite fragile and usually get damaged during the disassembly.
Nick