You can pick up braces online much cheaper than you can elsewhere. Sizing isn't a problem as online retailers instruct you on what measurements are required to pick the correct size.
Customer service at online retailers are used to enquiries and are also a good place to start.
When you buy a brace from a regular store the extra price covers things such as fitting and face to face instructions. You need to weight up the savings versus the loss of the extra service when buying online.
If a shop doesn't offer fitting then you will be much better off buying on the internet. The Brace Shop is a good place to get an idea of what is available.