What is this palette knife called, and where could I find it?
Painting knives don't seem to have much in the way of naming standards for blades. Most are just numbered in a given series. Some manufacturers do manage to group styles together but the names don't carry between. 

From what I can gather the very generic name for that is a offset trowel/spatula knife. Offset due to the crocked handle. The tips that are closely shaped to the one you have pictured with similar contours I have seen called flame and raindrop but looking up those would only yield specific manufactures. 



Image from oakblade.com

Diamond shaped blades are very similar but have harder edges and generally less curves. 



Image from oakblade.com

After reading one of your comments about it being "leaf" shaped I looked for that as well. The name is fitting but it does not look like any brands use that for their products. 

After a tonne of searching trying to name the thing I did manage to find one manufacturer. It looks like this blade is part of the RGM New Age line 



Image from rgm-art.com

So good news .... the answer to your question is it's a 3. Can't really tell you where to buy them exactly but you should be able to find a seller if you look for the "new age" line from RGM.

I did manage to find another ones of your knives in the wild. If you look at the post it is the 3rd knife from the left in the first picture. It actually looks like another knife from the New Age collection (I think No. 2?). I messaged the page owner to see if they could provide some insight as well. They will have more knowledge then I would as they actually paint with these.  

Could always make your own

Knives that closely resemble what you are looking are not hard to find. You could buy a knife that has a "flame" blade and use a dremel to bring in the curves on the end of the blade. Some fine sanding will make the new edge a little more gentle. 

Doing this would damage the finish on the metal. As you see on your knife in the picture that will happen naturally anyway and does little to the effectiveness of the tool.