Some tracks are album only on on the iTunes Store. They should usually say so where the price normally shows up.
If that is not the case, search for the song in the iTunes Store. Select it from the Songs List. Do not use the Purchased tab. Do not choose the album. When it is selected it will still show up a part of an album, but your chosen song's title will be blue. Tap the share icon as Johanna illustrated and (with a little luck) the Gift icon will be there.
Good luck.
Edit: Looks like I'm late to the show. It's all been covered. Ignore this. I'm just leaving it incase someone's already read and responded before I could delete it.