I've had some great experiences with henna - although I only use it to boost the strength of my hair, not so much for colour. You can buy it pretty much anywhere here (every hair shop, corner store, or market - but I'm not in the US for those that are).

I'd say, once you have a good formula, that's what makes the biggest difference. When I added extra conditioner to mine, my hair loved it so much and the washing out process was MUCH less painful. I also use black tea to release the dye, because I think it's acidic enough. Finally, I add in a bunch of oils including castor and coconut to give it a little extra conditioning.

Happy Henna-ing!
4A, super thick, fine edges - low maintenance

2.5 years natural (no way... really?) and loving every tangled, tousled, fuzzy edged, frizzy, curly, coily minute of it!

My Fotki Natural hair journey: http://members.fotki.com/Tellitv8/about/

My YouTube: https://www.youtube.com/user/TellyTV8