Are you looking for a day coat and jods, an equitation suit, formal suit, etc. Ebay
is one place to look but there is also showstoppertack.com, hartmeyer's. Unless you are looking for an equitation suit it is cheaper to buy a pair of black jods and get a day coat or two.
You have to be really careful of what you buy because the sizes are so different. We had a heck of a time getting my daughters clothes.
There is also another site, can't remember the name of it but it is for saddlebreds and they have clothes on there.