you can't (from my experience) get the actual plastic key/peg/tuner separately.
You can of course get replacement machine heads, in strips of three or singularly, depending on the design.
If you want to botch the job, then I suggest Sugru. It may work, but given the nature of a tuning key, it may require something more robust.
If you are not familiar, sugru is a air curing material that you shape to your requirements.
If you search google for machine heads, it throws up many VERY cheap sets on ebay. Personally I would avoid these. I use Gotoh, but they are high end and may cost more than the value of the guitar. Schaller are good quality and cheaper.