Second Helen's recommendation of SRB. But for those of us in the US, www.skgrimes.com may be a bit less expensive because of the unfavorable (for us) price of UKP.
I was able to find filter step rings that fit my Cine Ektars, so didn't have to buy custom rings for reverse mounting. Note the plural. I've tried 25/1.4 and 15/2.5 CEs. These two are usable, but the 25/1.9 is much better.