Ruth, my biggest suggestion is to find a reliable tailor. You're small, and the sad fact is that a lot of time clothing just isn't made for people quite that tiny. Obviously, you don't want to buy things that are two sizes too big overall, but if a piece fits well in the shoulder but needs to be brought in at the waist or shortened at the hem, a tailor can do that.
My next suggestion would be to find something really classic that you can wear to several different occasions. The obvious choice, I think, would be some sort of little black dress, or a dress in another color that you could wear over and over again. And if it doesn't fit perfectly in the store, buy it (making sure it is returnable), then take it to a tailor to see whether it can be altered to fit your frame. If so, get it done, and then you will have a reliable go to dress whenever you have a need to wear one. If it can't be altered, return it.
I think you need to have one dress (or outfit) in your closet that you feel looks great and can be worn to events where you have to be a little more dressed up (be it a wedding, cocktail party or funeral). If it isn't in your budget to do so right now, perhaps you can do it down the road.
Finally, you might want to check out Banana Republic's petite line (available online and in some stores). I was in the store the other day and saw a black dress on the sale rack in size 00 petite. Hopefully something like that would come close to fitting your frame.