The weekend before last I went to stay with the lovely YorkshireLass and we went for a little trip to a Haberdashers called Bond's on the outskirts of Leeds. Haby is the British word for Notions, and is all the zips, buttons, cord, thread etc bits and pieces. We spent 3 hours in the shop and spent quite a bit of money.
Bond's have some large bins stuffed full of zips for 20 pence each. I noticed that they had some coloured invisible zips (no black ones though) and spent time diving through the bins adding invisible zips to my basket in what I thought would be useful colours. I did go a little bit mad, and when we got back and counted them I had purchased 54 of them. Somewhat ironically I have mislaid the invisible zipper foot for my Janome so can't currently install any of them!
My local haby place sells only black or navy invisible zippers and they are £1.20 each, so I didn't feel like I was too silly buying all of these. They now have a special box to live in and I've added all the invisible zips from my generic sipper box to them, so when I want an invisible zip they will be very easy to find.