Trip to Haberdashery Shop
I've been going to Barnet Lawson, 16/17 Little Portland St, nr Oxford Circus for years.
When you get there, search through the list of businesses, when you see Barnett Lawson ring buzzer and they'll let you in.
It really is an Aladdin's cave of buttons, braiding's, lace, feathers etc. all at trade prices but also available to the public.
They have 3 sections with floor to ceiling shelving units full of boxes of goodies.
These pieces are silk frills with ready made button openings. They are a bit pricey at £14 + VAT per mtr but you could get away with buying 30cms (ie.about £4.50 + vat)and using them to create an opening in a top, which would save you loads of time.
The metalic lace in above picture is between £1.25 and £1.75 per mtr and is 5cms wide.Don't bother going to their website, as it has hardly any of the stock that's there and is not a good representation of what they do.
Have fun!