I got mine from Twinkle Twinkle: http://www.twinkleontheweb.co.uk/acatal ... 09EXT.html They fit most of our vests (Tesco, Asda, Mothercare, Matalan). The only ones that don't work with them are our Gap ones as the poppers are a slightly different size.
I had some, I think from babykind. Great with supermarket vests but anything slightly more expensive, especially bodysuits with dresses/dungarees always had funny sized poppers and didn't work