Well, I'm a HUGE proponent of adoption through rescues and shelters, but if I was going to buy a puppy, I'd choose a known reputable breeder. I don't think people who breed different breeds together are considered reputable. If you go to the French Bulldog Rescue Network's website, choose the Facts & Info tab, and scroll down about half way down- it gives some good tips about breeders.
I read an article once (sure wish I bookmarked it) that mixing breeds does not guarantee a healthier dog. The puppy can pick up positive or negative traits of either parent (say, the tendency for spinal issues from the Frenchie side, as an example) instead of only positive traits from both breeds as many people would think.
Puppies are hard to resist no matter what, and Frenchie/Boston mix puppies are no exception! If one showed up on my doorstep with a big bow around it's neck, I know I couldn't say no!