I tend to prefer bringing new blood in via hatching eggs. Relatively few diseases transmit through the egg to chicks, and many of those only have a small percentage of transmission that's even smaller if the birds are not outwardly symptomatic as I'd hope they would be if I'm being sold eggs from them. You can even gently wash the eggs before setting them to make sure nothing is clinging to the shell that would infect the chicks upon hatch. This does not guarantee that you'll prevent all disease from getting through to your flock, but greatly increases your odds at least.
If that's simply not an option, the best you can do is a 4-6 week quarantine. Keep new birds 100 feet away and downwind from your current flock, or if that's not possible then keep them in an enclosed building that your current birds have no access to. For example, I quarantine new birds in my basement because I don't have any outbuildings that are predator secure and far enough away from my current flocks. Always tend to the new birds last, and change clothes and shoes between them and your current flock. I prefer to outright shower immediately after dealing with quarantine birds just to be as safe as possible. I also introduce a 'sacrificial' bird to quarantine after a couple weeks, a bird I already have around that I may or may not be keeping anyway like an extra cockerel, to see if they'll start showing symptoms from being exposed to the new birds for a couple weeks. This also exposes the new birds to anything your flock may have, and if the new bird is a solo bird then it now has a buddy to integrate with. Like hatching eggs, quarantine is not a guarantee that you'll avoid all diseases, and it's a pain in the neck to keep up for the full time, but you at least have a better chance at avoiding your flock being infected by something new if you do it properly.
Very informative! Eggs is the only way I'd be bringing in new blood in, but I still worry about a few diseases