A forum member from here who is fairly local to me advertises on ebay that he has a VCDS cable and people can pay via a 'buy it now' price to have his services!
So why not give something like that a go as you can link to your website from the ebay ad.
It never ceases to surprise me the amount of people that buy just about everything on ebay and it has increased markedly since everyone and their dog has a smart phone. I've not looked on ebay for detailers so you may be unique. Don't forget to put your area in the heading though or you may be expected to travel the length of the country!!
Failing that, or in addition to, leaflet professional type companies in your area as a lot of high end car drivers are cash rich but time poor and would happily pay someone to clean their expensive car so they can look good down at the golf club or for ferrying clients about.
Local forums are good, once you have a few customers on a forum happy your name soon gets around, and people on forums know people not on forums and so it goes on...
A lot of people who are either lazy (like a lot of non car enthusiast drivers are) or just don't have facilities close to their home like to get their cars cleaned professionally before they sell them so gumtree, autotrader, local papers may be worth getting a small ad inserted.
Get people you know who work in large-ish companies local to you to put flyers up on their staff notice boards. Everyone who works in the place will see the advert and will pass info on to friends and family members too if you make your service look a bit unique and catchy enough for people to remember it.
I put up some details on our staff notice board when a guy I know locally started his own VAG indie garage, it brought a surprising amount of work his way.
There are probably a good few established detailing and valeting companies in your area so you need to be able to stand out from the crowd a bit so people remember you. A lot of people can't afford or justify spending a couple of hundred quid on a full detailing session, myself included, but as you're part time you could perhaps push things like paint correction at affordable prices as everyone hates crappy paintwork and smaller jobs won't take up so much time. Your prices look quite attractive though, so you'd certainly get people wanting to sell their car using you to clean it up and then possibly get repeat work from them to keep their new purchase looking better when they realise how good and affordable you are!!
