Hello M Smith! Have you looked in the SAA Catalogue? There are many very good offers for brushes, both single brushes and sets.
Brushes are often discussed, with the advice; "Buy the best you can afford!" and this adds to possible confusion.
A common sense approach to this question is as follows;
If you're a relative newcomer to Watercolour, then I don't think there is a need to splash out on expensive brushes. Save that for when you're decided and committed to a particular style and then you can consider professional equipment and materials.
I run Demonstrations around the North West and use a minimum of brush sizes; this suits me as I have a quite loose style, so I tend to use a 1" Flat for wetting, or large area's
A very good large round would be something like a No. 16 with, say a No.6 for general use and a No.3 for finer detailed work.
A brush I find very useful for fine work, such as finer tree branches, is a No.2 or No.3 Line Writer.
I personally like the Stratford & York brushes, and they are very cost effective.
All of the above are my personal choices, but they would get you started for not much outlay and give you a very good starting point.
I hope the above is of some use, but come back to me if you want more specific advice, and I'll do my best to help.
Very best wishes,
RSC Wigan / Warrington / St Helens