Just saw this thread - great topic!

I only recently (e.g. two months ago) began collecting them, but this is what I've found:

Swiss LX - Free after I asked (Y-Class).
Lufthansa LH - Free after asking (multiple decks provided, some odd kid-like motif. (C-Class).
Air New Zealand NZ - Free after asking, though not always available (2 out of 12 flights had them, Y and C-class).
Qantas QF - Free after asking (multiple decks provided, Y-Class).
JetStar JQ - Available for purchase on website only.
Virgin Blue - Unavailable in-flight, suggested purchase via online store which doesn't exist. Looks like ebay will be your only option.
United Airlines UA - Unavailable on any flight that I checked.
US Airways US - Unavailable on any flight that I checked.
Singapore SQ - Apparently widely available, not sure if based on class of service.

Looking forward to collecting many more in the coming months.

I was actually surprised at how many other transportation entities have them - on the same trip I was able to get cards for three different train services (all for a fee), and an inter-island ferry. I'm amassing quite a collection and I don't even play cards...