I'd really recommend you try the British Chihuahua Club, there are contact numbers on page below
They were very kind and helpful when I spoke to them not so long ago.
Please avoid the puppy sites, online and free ads, it sounds as though you will anyway
Buying via a BCC breeder means that you should get a better quality Chihuahua, hopefully healthier too.
Just my thoughts
Good luck and do let us know how you get on!