Swifts are particularabout their nest site.
If you want to buy one – either a box or a cup – then the RSPB is a good place to go. https://shopping.rspb.org.uk/bird-feeders-boxes-tables/bird-houses-nest-boxes/garden-bird-nest-boxes/rspb-swift-nest-box.html
If you want to try making one – Then there is a range of designs on the Bristol Swifts site https://www.bristolswifts.co.uk/swift-nest-box-design/ and there is also a link there for an MP3 audio file to play to attract them.