I was going to mention ebay as well. I found some really tiny stuff there when I got Chibi. It is better to find them where they give measurements as said above. I have bought xxs small that was still way big and then also bought them that was to small?
Another good site to check is etsy.com, similar to ebay but most shops are artists, etc and all goods are vintage or homemade. You can find really gorgeous stuff and almost everything is made to size, or you can request a custom size for her. I think most of the shops are based in the US and Canada, but most do international shipping regularly.
There are 2 awesome sellers on the uk ebay website that make gorgeous and tiny clothes
one is 1-2-3rt and the other is piggyb's
Just search 'handmade chihuahua clothes' it should come up
And also try urbanpup which is another awesome uk website and has sizes from xxs
US websites like g.w little and doggiecouture also delivery to the UK and they have GORGEOUS things
I can't help with where to buy one because I am in Australia, but I can give you a brand to look out for. I got Pebbles a fantastic coat made by dog-gone smart wear. At about 2lb it fits perfectly. The tag says it is an 8 which I am assuming means 8".
It has a website on the tag which is Dog Gone Smart Beds . Might be worth checking if they have a distributor anywhere near you or if you can order online. Or if Dottie is going to stay small I can send you Pebbles' after this winter because it won't fit her for too long You can never have too many coats lol