It's very very hard to find reputable breeders of crosses...the thing is that it's hard to find good breeders of pedigree dogs and there are lots of reasons to be breeding them other than money, so to find one for crosses is even harder.
A reputable breeder should be breeding for a reason other than just selling puppies - and there aren't that many reasons to breed crosses, so that limits your field straight away. (In my opinion breeding a cross to sell them as nice pets isn't a good enough reason to add to the dog population - there are thousands of perfectly nice crosses already in existance in rescues)
They should do the health tests appropriate for both breeds they are crossing. Many don't bother, they talk about things like hybrid vigour and crosses being healthier when that isn't necessarily the case at all.
They should also interview you to see if you're suitable owners, insist on meeting you all and insist on taking back the puppy if you ever need to rehome it.
Reputable breeders breed rarely, once every couple of years - because they're not just churning out puppies and they usually have a waiting list of owners before a bitch is even mated.
Things like insurance and a puppy pack help as well, but, even some dodgy breeders do those.
See if there's a club for the cross you're interested in - its probably your best bet for finding a decent breeder. Of all of those, I wouldn't touch a breeder who hasn't done the health tests, not just a vet check, but the proper tests appropriate to the breed with certification.