Hopefully Barry will make this a sticky at the top of the forum.
These are not in any particular order. They should be considered as potential sources of varroa tolerant queens. Whether they are reliable businesses is up to you to investigate. Purvis and Carpenters in particular impress me. Both have developed businesses based on producing survivor queens that can produce a crop of honey.
http://www.dixiebeesupply.com/ This is fatbeeman's website. He is more of an eco queen producer than a pure survivor queen breeder.
I am going to edit this post repeatedly as I find articles on breeding for varroa tolerance. Here are a couple to get started.
The known mechanisms for varroa tolerance include:
Varroa Selective Hygiene - disrupts the reproductive cycle of the varroa mite
a. Detect infested larvae
b. Uncap infested larvae
c. Remove infested larvae
d. selection involves testing for hygienic behavior and removal of infested larvae
Allogrooming - bees grooming each other to remove mites
a. Varroa mauling - chewing and biting the mites which kills them
b. Selection involves monitoring for chewed mites on the bottom board
Breaks in brood rearing - during brood breaks, varroa cannot reproduce.
a. Heavy pollen collection - bees that collect pollen heavily are more sensitive to lack of pollen and shut down brood rearing earlier.
b. Sensitive to nectar dearth - bees that react to nectar shortage by breaking the brood cycle.
c. Selection involves monitoring for bees that reduce brood rearing when pollen is unavailable.
Reduced days to worker maturity - fewer days gives mites less time to reproduce
a. some worker bees mature in 19 days vs standard 21
b. using small cell foundation and timing brood emergence
c. Selection involves identifying the small percentage of colonies that mature workers in fewer days.
The major hurdle is that varroa tolerance so far has come at the price of lost honey production. It is relatively easy to breed for bees that are highly varroa tolerant. It has not been simple to combine the various tolerance mechanisms with production of high average honey crops. In some cases, VSH queens are too aggressive at uncapping brood. Bees that reduce brood rearing tend to be unadapted to areas with both spring and fall nectar flows.