Hmm guess you need to practice you web search skills.
Putting "french knickers" into Google brought up pages of suppliers.
Also whilst you won't find them on the mainstream high street any more they are readily available from stalls at the monthly fetish fairs if you don't want to trust your plastic to the world wide wibble.
Finally of course there are shops like Fairy Goth Mother in Camden market.
For suspender belts and the like try What Katie Did in addition to the knickers suppliers.