Have you checked out whether you can get one on the NHS? See https://www.nhs.uk/using-the-nhs/help-with-health-costs/wigs-and-fabric-supports-on-the-nhs/ They are available free for a number of conditions depending on whether you are on any kind of benefit.
When my youngest was on chemo she had one lovely real hair wig paid for by the Teenage Cancer trust which was great because we could straighten it curl it etc
BUT she had many many cheaper synthetic wigs too bought all over the place including ebay. Once she had got over needing one (she was just 16) she actually had a great time different styles colours and lengths,
In your situation though it may be that you want one really good wig so that no-one knows it isn't your own hair. It was actually a hairdressers who made my daughter's quite locally for her so could that be somewhere to start?
Good luck and take care