It's not really a detergent as such in standard form, its just a foaming agent to cling to and hopefully pull the grime from the car so you can then minimise the risk of scratching when washing.
Some of them do have detergents and nice smells added to them though, at the moment I just add a bit of shampoo in there to give a bit of cleaning as well as filth dragging.
I get mine from here:
http://www.cleaningproductsuk.co.uk
via eBay:
http://stores.ebay.co.uk/AUTO-RAE-CHEM
They do the citrus version that has a detergent in it but you may find this also takes a bit of wax with it whereas the basic stuff won't. I just mix a bit of Zymol autowash in with mine, smells pretty too
The same company also sells the foam lances for use with the power washers, I have the HD one they advertise and it's huge amounts of fun, great thick foam.
http://www.megaboost.co.uk/supra/mine/detail-14-10-07/P1100357.JPG
http://www.megaboost.co.uk/supra/mine/detail-14-10-07/P1100361.JPG
You still need to wash the car afterwards if you are doing a proper job of things but if it's well prepared and waxed up you can get away with a midweek foam and power wash and still get a nice finish without going near it.
An alternative to the lance above is something like a Gilmour Foamaster gun, I have an original one of these too but don't use it now that I have the other one, gives a nice foam but not quite as thick - you can buy it from me if you want £35.00, it'll just fit on the end of a normal hose, if you have decent water pressure you'll be fairly happy