Microdrives are obsolete as you can get compact flash cards of 6GB and more now! Anywhere that sells digital cameras should have compact flash though some places only sell up to 512 or even 256, I dont buy higher than 1GB anyway in case one fails (has happened) 4GB of pics down the drain would be a nightmare.

The best place to get them is on the net, they are very cheap now and if your camera isnt new (bought in last year) then you can pretty much ignore the write speed of the card unless your camera can take eight shots a second or you like to tie a rubber band over the shutter when something interesting is happening :)
1GB for £57 here:
http://www.pixmania.co.uk/uk/uk/50544/art/pixmania/compact-flash-1gb-card.html

You could probably can get cheaper though with a bit of searching.