Good question. I don't know about salon colour but I *think* Herbatint, Sanotint and Naturtint are PPD-free. I've ordered these from here (if you live in Canada): http://www.aviva.ca/shop/product_sections.asp?catid=238
Surya Henna is also supposed to be good (Pure + Simple in Toronto carries it) but henna is tricky to work with.