I have had mixed results over the year, i would buy off some one who's only selling occasionally (you will see on feed back( can some times see hatch rate also)) that way you will know they had been hatching for themselves and only selling the surplus few, secondly you can search by distance to your location - try finding some one close and collect this would hopefully improve the hatch rate.
The post definitely changes things i have had eggs from the same breeder twice over the period of a month and a half, first one i got 3 out of 12, second i got 10 out of 12 - i know the person and they were out of the same pen who was getting 8-10 out of 12 for her hatches.
It is very variable on breed / cross some more highly bred birds seem harder to get fertile and hatch whilst your cross bred with hybrid vigour may hatch after half a dozen power cuts - no rhyme or reason. If the advert says they are using breeders pellets that's a bonus also. Don't pay silly money for eggs, i always work it out that if i got 1 out of 6 would i have payed that for an adult bird of that breed/ strain that means anything else is a bonus. You are defiantly taking a chance but its a good way to get strains that aren't near you
I always rest my eggs for a day ( don't know if its an old wife's tale but i do it anyway)
Also buy more than half a dozen - a dozen / 2 dozen ( get them to send a doz at a time as otherwise they will be sitting to long) - that way if 1 hatched per 6 you at least have 4
I have big light Sussex but breeding pens are done for this year, (Plenty of growers but i am in central Scotland a bit far from you) if you are still struggling let me know next spring if you are still looking for hatching eggs out of a fertile pen.
Also Re golf ball eggs there are a few big breeds of hens that will lay them i have hatched them before not what you want for selling eggs but a few heavy breed have a tendency to produce these
Cheers
BL