For your convenience: good gold leaf supplies (and other metal leaf) from reliable US-based suppliers.
(UK- and Europe-based gold leaf supplies here.)
Everything below is from vendors I can vouch for. Two (Blick and Amazon) are old friends now who give me small commissions on sales through this site. The others are well known and trusted either from direct experience or because close artist friends have used and recommended them.
Note: always read reviews/answered questions where available. Also note: links below open a new window each time so you can compare products side by side. If you don’t want that, just start new searches in the first window that opens for a site.
Dick Blick is a reputable, friendly, fine-arts and crafts supplier with an easy-to-navigate website, well selected products, frequent special offers and good customer service. I like Blick.
18k gold leaf (loose) – NB the image off this link shows white gold leaf but if you check the product description it is actually yellow 18k gold.
22k gold leaf (loose) – good value but not much cheaper than the next one, 23.75k, which is 99% gold.
12k white gold leaf (loose) – NB this has a warm silvery look, but unlike silver does not tarnish: it’s half gold, half other metals alloyed to create the silver colour, the same as the white gold used in jewellery.
Blick also do Roll Gold, which I’ve not seen elsewhere: it’s a 67-foot roll of 23k transfer gold, ¼” or ¾” wide (check which width is available), easy to cut and handle, very useful for borders or lettering.
Blick stock both Mona Lisa and Wehrung & Billmeier imitation/composition gold leaf. It's gold(ish) coloured, and it's cheap. Note: you have to varnish it, or it tarnishes streaky/dull – so, not obviously well suited to calligraphic work. Possibly worth experimenting with spray varnish if your media will take it.
Pebeo Gedeo 'Mirror Effect Metal Leaves' – described as suitable for application to almost anything and apparently won't tarnish. Not for high-end use! It’s cheap and it's very very shiny. I’ve linked to the gold version but it also comes in (intense) red, green or blue as well as silver.
There are several other of these kits chez Blick in different sizes containing imitation/composition gold, silver, or copper leaf. Easy way to find them is to explore from one of the above links.
Good quality products, sound packaging, speedy delivery and efficient customer service. Yay! That said, the gold leaf supplies are fulfilled by third-party sellers: check before ordering whether products qualify for free shipping.
Price comparison is the thing, and since new products and sellers arrive all the time, it makes sense to give search result pages not specific products ... do sift through for the best deal. (I worded the searches as carefully as possible but you know how it is: some irrelevant product always creeps into the main list ... and check the ‘also-ran’ products at the bottom of the page for overlooked bargains.)
Edible gold leaf falls between 23 and 24 karat – perfectly good for calligraphy, and doubles as deco for your hand-dipped truffles.
Suppliers of a variety of metal leaf including aluminum, copper, and real silver. NB they also stock two sizes of shell gold. Natural Pigments is a wonderful place for old-fashioned pigments.
Kremer's gold leaf and shell gold range is very good. Not cheap but excellent quality.
Kremer also do real silver leaf both loose and transfer.
Lastly, if you want a manual, Gold Leaf Techniques is a great reference tool (and a fascinating read).
Let me know how you go.