Gold leaf is available online. Try some gourmet food supply sites - I always get food grade gold leaf (because I use it for cakes and things). As for proper application, I'd suggest just reading the manufacturer's directions.
Rio Grande has gold leaf prices per piece. https://www.riogrande.com/product/23k-yellow-gold-metal-patent-leaf/681128GP/?code=681128
Amazon has food grade sheets - they're small 1 1/2 inch square or so, but if you're using it for jewelry, that's probably a perfect size.