Hey, glad to see more NI people ! i'm in Belfast, not sure if this would help but the Rusty Zip in Botanic here is a great place for thrift store clothes, and if you fancy early friday mornings St Georges market is a great place for bargains.
an embellished guitar strap could be quite easy if you buy a basic plain coloured one in a music store you could use fabric paint and stencil friend specific motiffs on to it or you could sew buttons, sequins etc. if you want to make the whole thing from scratch the canvas webbing tape for apolstery would be quite sturdy. i cant remember exactly where i saw them but elsewhere on the boards here there have been some cool ideas so do a search of the site and that might help you.
Hey one of the sites i like a lot is jilly beads http://jillybeads.co.uk/acatalog/ its not a massive site but they've some great stuff, also if your willing to go further into cheshire there's a hobbycraft store in Stockport, and a couple of smaller crafty stores in the town centre.
Hey, you've got some really nice stuff there ! I was in my local supermarket yesterday and the girl queueing(sp?) in front of me had a great beanie hat that she'd edged with all different shaped and coloured buttons, it was very simple but i thought it worked well