My understanding is that it is only the "pure" endlers that are hard to come by. The endlers crosses are common a lot of places. part of the problem with endlers is they weren't recognozed as being a seperate fish for a long time, and therefore were crossed into Guppy strains quite often. The result was a lot of cross bred fish but a lack of good strains for breeding pure fish. It is close to impossible to tell the difference between female endlers and female common guppies, the males are distinctive, but also similar enough to be readily mistaken. Their breeding and activity habits are also very much the same, so mixing is extremely common while at the same time, endlers purists can't find good stock when they want it. In the old days I just thought common guppies looked like that, as I've learned more and paid more attention, I've found that feeder gupies are quite often endlers cross fish. Almost all guppies in captivity are reproductions of tank raised fish, and the Common guppies used for feeders are repetative often cross bred fish.
My own experience has found what seem to be some minor differences, none of which have I tried to document fully or researched for accuracy yet.
Endlers seem to reach maturity faster, and reproduce faster. when compared to fancy guppies, my feeders gestate in about half the time. also the males pick up color and start exibiting courting activity in 3-4 weeks while guppies seem to take 5-7 weeks for the same.
the most pronounced difference is size, especially with the females. most guppies I have grow to fully twice the size of the endlers crosses i get. I used to attribute this solely to the larger size of fancy guppies, but I tend to see far more size seperation whenever the endlers colors are heavily present. The endlers are not as good for feeders IMO solely because of their size limitations. but I still keep a few around because they are more prolific and fun to watch. As said, these are just my observations and have not been tested properly so they me be partially or even completely off.
dave