The database here is pretty good, but is incomplete for many genera. It's a good starting point and if there isn't a plant similar to what you have in mind in the database here already, the next step would be to check with the corresponding ICRA.
This site is a list of the known ICRAs by genus.
https://www.ishs.org/sci/taxli...
For example, if you go to "D" and select "Dahlia", this is the entry:
https://www.ishs.org/sci/icral...
That would then take you to the RHS Dahlia Register, where you can download the current list of registered hybrids and also register your own:
https://www.rhs.org.uk/plants/...
