Hi,
I am afraid that RS, Farnell , Digikey and Mouser are of limited or no use in providing ferrite cores for SMPS.
This is because you might be able to find a particular core size, but the chance of finding the corresponding coil former to go with it is about zero.
-also, finding a particular core set, ungapped is easy, but ungapped are less useful for flybacks where a gap is always needed.
In fact, a gapped core set, is obtained by using one gapped core half, and one ungapped core half....but the chances of finding both of these from RS, farnell, Digikey or mouser are zero.
Ferrite core sets..with corresponding coil formers, might exist in catalogs of companies like Epcos etc, but to the average person working in a small company, they are unobtainable no matter what price we are willing to pay.