For the Buck, contact Buck Knives. They've got an excellent reputation for customer service, and I'd bet they will send replacement screws to you. Several reputable makers of other knife brands do the same for their knives as well. It's an easy way for them to build customer loyalty.
It's often difficult to exactly match aftermarket screws to the originals, which often may be proprietary to the makers (designed to their own specs). Not just the drive size (such as T6), but the length, head diameter, shank diameter, thread diameter and thread pitch all have to match up, in order to work correctly (and might void the warranty, if they're not the right part). Best bet is to contact the makers of the knives, and see if they can do something for you.
Edited to add:
The thing I noticed, when trying to find some aftermarket replacement screws for my knives, was that many aftermarket sellers are wholesalers, and may require purchases in bulk (like packs of 100). Not necessarily that expensive, but still beyond what most will need, and still a big gamble if one's not sure if they're asking for the correct part. Contrast this to asking the knife's manufacturer; they know exactly what it needs, and may not even charge you for it. Might not even have to pay for shipping.