Cannot buy custom cables? like maybe fsp or cosair de can fit etc
Superflower PSU side modular cables use their own square 9 pin connector, which I dont think any other brand(s) are using so no you cannot buy a a "custom" cable from FSP or corsair and make it work.
Anyway those are not exactly "custom" since the lengths and type of cable/sleeving are standardized, custom would be making the exact length that you want/need.
If you want I can probably make some real custom ones for you(length/sleeving/etc), planning to make some for mine too(superflower modular psu).