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,

Steve Williams
RSC Wigan / Warrington / St Helens