Text to Speech
On Report a Crash I first got into using text to speech using this website. You can create whatever lyrics you want, and edit those to your comfort. Later on with Who let the Yeti out I used this program. The quality of the vocals on these websites are usually quite bad, so you have to put quite a lot of effects of them so make them sound decent. Problem is, sometimes you don't want spoken vocals, but sung.
Create your own vocals

A cappella's
So you want to get some already made vocals. But where to get these vocals? Some websites offer free a capella's, you could use google, or get some home sung. Other websites offer paid acapella's, but be careful though, a lot of them are low quality and therefore not very usable. It's can be quite hard to find an a capella of a specific song, so you might have to browse through lists and lists to see if there's anything that fits you. Another possibility is to do it yourself and, like always, there are Youtube tutorials for it.
So finding or making vocals can take quite a bit of work, but once you've finished your track it's all worth it. Even if it's just a couple words it can really give your track that catchy hook that will make your listeners remember it.