There are a lot of reasons why men's clothing from that period and of that style is hard to find. To begin with, it was a small faction of the young male population that bought the most outrageous clothes. The most flamboyant pieces were bought by gay men, musicians and artists, and boutique fashions were notoriously poorly made - the opposite of a Saville Row suit - and tended to fall apart after one or two dry cleanings or washes. Men are generally not as sentimental about their clothes as women and tend not to keep them but those pieces that do survive are still likely in the possession of their original owners or, because they have been popular collectables for over a decade, they have been snatched up and squirreled away into museum and private collections already - there is a lot of competition for the best pieces. Many pieces from that period are not conspicuously stylish - a polka-dot shirt, turtle neck sweater, or a pair of skinny legged chinos can look very unremarkable on a hanger and overlooked in a thrift shop or vintage clothing store. Only when those pieces are styled with a mop-top hairstyle, love beads, granny glasses, a kipper tie, or velvet jacket that they become noticeably 'of the period'. If you look for some of the more popular and available brands from then, like Ben Sherman and the various John Stephen labels, you will find those more readily than the rare Granny Takes a Trip labelled clothes. Also some of those stores, like Lord Kitchener's Valet, didn't label their clothes as far as I am aware - they were a vintage clothing shop only and sold things like Victorian military and livery coats for that Sgt. Pepper/Jimi Hendrix look.
