In most towns the UK we have hardware stores such as Screwfix and Toolstation that stock nut bolts, screws, etc. You can buy, for instance, 6mm stainless nyloc nuts for less than 4 pence each in a bags of 100. I usually buy stainless nuts, bolts and washers like this so I always have some "in stock". You won't find any Imperial threads such as BSF or Cycle but quite a few bolts on a Vincent can be replaced cheaply with stainless metric. If I need stainless Imperial I get them at Classic shows or autojumbles where there are usully several stalls selling, at a premium, this kind of hardware.
I hope I don't get drummed out of the club for using metric or stainless fixing on my Rapide. Following this revelation, I will be hiding behind the settee if I hear the originality Gestapo banging on my door.
Just stop all this defeatist nonsense have a look at Carbon Engineering Ltd a much more sensible solution to making current transport systems zero carbon (a zero carbon Vincent!!!) and with some tweeking negative carbon for the world now that will upset the greens and the oil companiesI confess having changed most of the screws and bolts to metric, using lots of helicoils when necessary. No wish to deal with funny imperial sizes of various standards - a real mess - and a PITA to find fasteners today in those standards at reasonable prices. In 20 or 30 years nobody will likely care about what we have built into our bikes when you won´t be able to use them on the road then. Environmentalists will see to that or so it seems today.
As to thread locking, I don´t care about nylocs, don´t like the looks of them and tensile strength is not always known about these. Instead I apply some Loctite in carefully chosen types according to my desired safety requirements. When pairing SS screws with SS nuts you will definitely apply Loctite or alternatively anti-seize at all times on threads. Otherwise you´d be getting seizures in the lot and loads of foul words later.
Vic