A couple of booksellers I can recommend: https://www.abebooks.com (good for rare and hard-to-find stuff, but can be expensive) https://www.biblio.com (used and rare books, also expensive depending on edition) https://www.peterharrington.co.uk (UK seller of used and rare books, boutique, expensive, but good inventory) https://www.bookfinder.com (aggregator of booksellers) Abebooks would be my first port of call, then I'd try some of the others. UK booksellers tend to be good options for Anglican-specific stuff (as you'd expect), but you may have to pay extra fees and duties if you're shipping to the US.
You could purchase it online from the Prayer Book Society (https://www.pbs.org.uk/). It can also be purchased online from its publishers Cambridge University Press (https://www.cambridge.org/) and Oxford University Press (https://global.oup.com/). Online booksellers will sell it such as Amazon (https://www.amazon.co.uk/) and AbeBooks (https://www.abebooks.co.uk/). Booksellers such as Blackwells (https://blackwells.co.uk/bookshop/home), Waterstones (https://www.waterstones.com/) and WHSmith (https://www.whsmith.co.uk/) will either stock it, be able to order it for you or sell it online. Any bookseller who does not have a copy in stock would be able to order it for you. Basically, you are spoilt for choice.
I ordered mine through a local "Christian" bookstore. But the offers above look like far superior options.
I bought my international edition through Indigo/Chapters, being the best option for cost in Canada compared to ordering from IVP online and shipping from the US.