Another helpful location for bronze fastners is mertons fiberglass and marine supply located here Merton's Fiberglass & Marine Supply
The owner, Joe Merton, was very helpful. I changed out 2 seacocks last year (for the first time) and have another 2 for this spring. It is not that difficult. I found the removal of the old seacockcs the most challenging but Maine Sails' information was most helpful for confidence with installing new ones. It was quite exciting when we splashed and there were no leaks! The seacocks in my 34 year old boat just used stainless steel 3/8" by 1" long lag screws into the raised bed. With the ones I installed I went with thru bolts but not sure that it is entirely necessary. That is the seacock and thru hull are screwed together with the beds they rest on are thick enough that the lag screws seem to offer resistance from spinng free not to mention the caulk also keeps things in place so not sure thru bolts are necessary. anyways just something to consider. Use a long enough drill bit to set the thru bolt and maintain the angle to avoid hitting the outside edge of the thru hull. I drilled from the inside out. Also have enough length of the bolt to accomodate that angle do not just measure bolt length straight down from inside the hull to outside.