Where to find older sets (eg. Classic Space)?
Some of the places you can buy old (discontinued) sets include:

Online 


BrickLink
eBay
TradeMe
Amazon marketplace
Generic on-line toy stores that carry old stock
Some LEGO discussion forums have areas for buying and selling


Locally


Toy shops and LEGO stores sometimes carry old stock
Stands at exhibitions and shows
Garage sales
Thrift stores
Charity shops
Flea markets
Car boot sales
Jumble sales
Through newspaper ads
Lego User Groups
Other AFOLs


Other Alternatives


Buy the parts separately and assemble the set from them