Some manufacturers, eg. Revell-Monogram, provide online instructions for their current in-production kits. Click here
For Asian kits - eg, some (but not all) Dragon and Tamiya kits (and others), instructions may be found at http://www.1999.co.jp/eng/plamo/ This is an online retailer, but they have scans of the instructions for many of their kits - search for the kit required and you may find the instructions.