Nice to hear from somebody who wants to attract, rather than get rid of them! Lots of people don't like the mess on their windowsills. But it is lovely to peer out of an upstairs window and see/hear the babies.
Anyway, You can get the manmade nests from CJ Wildbirdfoods (www.birdfood.co.uk). They have both swift nest boxes and realistic dome-shaped martin nests that sit under the eaves. Good luck, hopefully next year you'll be hearinghave the flutter of tiny wings !