Question: What is the standard practice if I can no longer find students that contributed to a paper that we would like to publish now? The students contributed data, but obviously had nothing to do with writing the paper. Still, I feel that they deserve authorship. I can no longer find a current email address for them. Should I put them only in the acknowledgements section?

Answer: 
This is a tricky one! Since some venues may require each author to sign over copyright, it complicates matters above and beyond the ethical question of what is right. 

If you feel they deserve authorship, you should IMHO make a good faith effort to find them (web search, ask the university for contact info, etc). But in the end, if you can't find someone and they contributed data but not writing the paper, I see no way you can include them as an author. An acknowledgment would be appropriate.
This is a difficult one, but I think in the end you have no choice but to put them in the acknowledgements if you cannot contact them, with an appropriate note regarding the level of their contributions. **You should do your best to find them** (social media? Contact other past students? Google?), but sometimes this might not be possible.

**It is assumed that any author on a paper has given their consent to have their name attached to that particular piece of work.** In your situation, the student(s) have not had this opportunity. I would be (rightly) upset if I found that someone had given me authorship on a paper without my consent. It would probably stop me doing any work with this person in future, and if I didn't like the published manuscript it would also involve a letter to the journal editor explaining the situation.

Even worse (I'm not suggesting this is the situation in your case), if the paper is of dubious quality, it may even hurt the student(s) in their future academic life to have their name attached to that paper.

Finally, I think most journals will have you sign off that each author has given their approval to the final manuscript. **Are you happy lying about this? If this is discovered, prepare to face retraction of the paper.**

I think it's a sign of a good supervisor that you value their contribution enough to want to give them authorship, even though you cannot contact them about it. It's just a shame that the rules of authorship preclude this.

Good luck with the paper!