I do not think there is a blooming list that can answer your question. Hopefully someone that has planted R. E. with a rhodie that blooms at the same time can help.

If not, R. E. should be very easy to propagate so try propagation.... or another rhododendron catawbiense.

http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=267884

https://landscapeplants.oregonstate.edu/plants/rhododendron-catawbiense-boursault

I agree with Luis, doubt you will find any listing that is that specific. The bloom time of any cultivar or species of rhododendron can be influenced by location , siting/exposure and local weather. Even all varieties of R. catawbiense do not necessarily share a simultaneous bloom time!

Best to look at what other rhododendrons are blooming at that time in your area but even that is not a 100% guarantee, for the reasons stated above.