It is a good idea to buy props well in advance. If you wait until the day before to buy that vital UV pen, you will find that every branch of Ryman's and Maplin has run out of them.
However, you may well find that you need to buy props at short notice, so this page contains details of bricks and mortar retailers. It's also worth browsing in a physical store to get ideas.
US web retailers
http://www.spygear.net/ has a lot of spy-themed children's toys. The 'night vision' products are basically head-mounted torches, rather than consumer-grade NVDs.
UK web retailers
Witzigs sell playing-card-sized blank white cards for £5.69 per 200 here, which actually work out cheaper than normal playing cards (and much cheaper if you order in bulk).
London prop suppliers (bricks and mortar)
- Hamleys has these as of June 2009.
- Good comprehensive range to buy or rent: Angels Fancy Dress, 119 Shaftesbury Avenue, WC2H 8AE
- http://www.fancydress.com/content/london_store (as of June 2009)
- Party Superstores at Clapham Junction; full-on costume rental, plus a lot of cheap props and hats and makeup
- Steampunk/vintage costumes: Prangsta, 304 New Cross Rd, SE14 6AF
- http://www.prangsta.co.uk (trading as of June 2009)
Electronics and technical
- Maplins is a good bet.
- Kleins on Noel Street turned out to be the cheapest, when I last did a haberdashery run.
- Hamleys is worth wandering round for inspiration until you get a headache. There's an amazing number of cheap toys which suggest gimmicks for games. The staff are not very good at knowing what is actually on sale outside of their little department.