Chipps found the ultimate source of plugs on Fresh Goods Friday the other week – £6.99 for 50 that are long enough to cut in half!
http://www.ebay.co.uk/itm/142118522716
You need to be a bit careful with the eBay ones as a lot of them are designed for car tyres and it can be almost impossible to cram them into a small-ish hole, unless you also have a reamer. 3.5mm is the width to go for.
The Dynaplug thing is meant to be great, but you need to be careful if you’re using the pointy plugs – they’re easily sharp enough to pierce an inner tube if you have to put one in later.
This looks good too: http://sahmurai-sword.uk/