i got mine at home depot. with liners, 3/4 is perfect.
the depot didn't have 5/8 or i would have bought a handful of those.
the FAQ/DB has the specs. that's why it's there, people keep asking
exercise: goto search and type in "becker stainless" or "becker screws"; enough hits to be useful
also: there have been reports of other screw sizes sometimes coming from KaBar lately, i'm not sure what that's about and have never managed to ask, but they're of a similar size.
worst case, when you buy screw/nut combos, always buy enough to replace everything, OR bring an existing screw/nut with you and buy exact replacements when testing at the store.
i have seriously considered just taking a spare screw/bolt and putting it onto a keychain through the hole of some key or whatnot and threading a few extra nuts as well... so i always have a spare, and have some way of measuring hardware if i forget the numbers (which i do :>)
one of these days, i'll find [annodized] Ti hardware and buy that

i'm sure some store sells them, but haven't seen them in person yet. heat treating those will make them turn funky colors.