Mains frequency transformers can be made in standard ranges because the input voltage and frequency is known. You can simply plug them into the mains and they will work. The only parameters are really output VA and voltage, so manufacturers can make a range of 10 or so sizes for different VA ratings, and then wind each size for 5 or so different voltages. That gives them 50 or so different transformers that need to be stocked.
SMPS transformers have so many parameters that they are usually made to order.
You can buy the components for transformers from the usual electronic suppliers. You need to chose a core, and then decide if you want it gapped (probably useful for a flyback SMPS) or not (more likely for a push-pull SMPS). The manufacturers who make the cores normally say how to calculate the inductance and the maximum magnetising current, and that lets you calculate the primary and secondary turns you need. You then have to decide if you can fit them in, and how the winding should be constructed. You can use wire or copper tape. Copper tape is used for higher currents. The winding can be secondary first, or primary first, or some of the secondary, the primary and then the rest of the secondary. You might need or want multiple thinner wires on some windings. Stray inductance is really important and the driving circuitry may have to be designed for the stray inductance of the transformer.
Many SMPS need more than two windings. There can be multiple outputs, and there can be an auxiliary winding to run the control circuitry.
From the same manufacturer as the core, you can usually buy a coil former and a clamp to hold the whole thing together.
I'm sorry if this doesn't help, but SMPS design is an art in itself. Unless you have a really good reason to build one, it is easier to buy a complete one.