If you're after one right away, try Eurocarparts.
https://www.eurocarparts.com/
There's usually a branch fairly locally, just enter your reg number for the ones that fit.
Look out for their discount codes, their current one is Box90 and will bring a decent Bosch S3 (3 year warranty) down to £58 and a Bosch S4 down to £74 (4 year), the S4 would be my choice.
Go through the click and collect and they'll reserve it to pick up, they do delivery too.
If you want to mail order one, try Euro's sister site, carparts4less
https://www.carparts4less.co.uk/
Prices are around the same as Euro's discounted prices.
Steer clear of the cheapo batteries, they'll work ok but don't tend to last.
Halfords put big mark ups on most of their stock, only use them if you are desperate.
