I have bought a few sets of misc Mini type nuts and bolts via eBay from British sources. Over the course of the build I've bought 2x sets of high tensile and 2x sets of stainless.
Eg one set "SENT: 363 PCS CLASSIC MINI COOPER UNF BOLTS KIT & NUTS & STAR LOCK + T4 WASHERS" but they came with star lock washers.
I preferred an earlier set I got from this bloke as it include split washers. Heaps better.

Edit: this is not a set of one of everything you need. But it includes virtually every nut, bolt and washer of the types needed. Best decision I made was to buy these kits. You have to source your own self tappers but those are available everywhere. And some specialty items like those used to secure the inner door handles or the window winder mechanism etc etc. Which you get through Minisport, MKO etc
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