You may want to look at some of the Power Integrations documentation, wizards etc. It may help you understand something about the 'custom' nature of trafos for flyback designs. As others, I don't recommend rolling your own or dicking around with mains but if you want to learn, their stuff/ref designs etc deserves recommendation.
As to your actual question: Wuerth and others have a small collection of 'standard' trafos but a major advantage of rolling your own is that you can make the trafo (and everything else) fit your application better. You'd obviously loose that advantage, so your delta with repurposing something pre-built decreases even further.