Tazz, I had the same problem - the La Redoute catalogue was my saviour! The clothes aren't expensive (some of them are even cheap), and they fit! Unless you're very short that is. Good luck.
Be proud of who you are. Size is unimportant just concentrate on being healthy rather than a certain shape/weight.
Gap clothes are a range of sizes and I'm sure you could find something there (plus the staff are far more helpful than in most shops so you could ask their advice- they tell you if things fit or not and bring you different sizes to your changing room).
I'm 5ft 8, and just about everything fits from La Redoute, but sometimes the skirts/dresses look longer on me than they do on the 9ft tall model in the catalogue! You should be fine - and as Poppy says, if there's a Gap near you, I've heard lots of people say they're good too.
Topshop does size 6 clothes. Other than that I think maybe you should see your dr if you are underweight and trying to put weight on and not managing it, it could be a symptom of diabetes
TK Maxx do size 6. Just to check your thyroids are ok pop into your doctors that could be a reason you can't seem to gain weight. Otherwise you are naturally thin make sure you keep above 2000 kals and don't be tempted to eat junk to put the weight on it will just damage your health.
Going for a vintage look might be the way forward - sizes were much smaller years ago. Ebay and vintage shops have some great items, from basics (I got a Jaeger jumper for �3 in a vintage shop) to more glam going out dresses etc.