Well,it depends on how dark your hair is. If its darker than a medium brown, u may want to consider lightening it first, otherwise u aren't going to get a good result.
I would say your best bet with purple hair dye would be a brand called Special Effects. Its not in many real stores that u can walk into, but many places sell it online. I would suggest going on ebay and doing a serach for something like "Purple special effects hair dye", and it should come up.
Other than ebay, u can get it (Depending on your location, anyway) from places such as:
link link u can also get it from this site-
link -, Which also has a lijst of all the colours door Special effects (Including the various purples), and includes pictures for most of the colours.
I have used special effects hair dye before, but none of the purples, sorry (Only pink). I have heard that the colour "Pimpin'Purple" is pretty good, though (
link)
Hope this helps! :)
Maisy xx