Come on, Google is your friend
Type something like
wooden door cill into a search engine and you'll find sites like this:
Shop external timber door sills at Love Skirting. Premium oak, pine & sapele sills machined in the UK. Free delivery over £999.
www.loveskirting.co.uk
And for fitting advice, type
fit wooden door cill into YouTube and you'll find quite a few videos offering methods to replace.