There are a few ways that you can get that manual online. For starters you need to know the brand and model number of the product you want a manual for. Further you can save it on your hard drive. An advantage of saving it on your hard drive is that you can download manuals for items you own and save them on your hard drive. You can save on storage space since the manual that came with your purchase can probably be discarded.
Many companies on their website (usually a customer support section) have a way to download the instruction manuals from their website to your hard drive. Two good websites are Manualsonline or Safemanuals. By creating a free account on Manualsonline you can have an online "Virtual Filing Cabinet" where you can store your downloaded manuals from their site. Further you can participate in discussion groups and receive personalized updates. The only downside to this virtual filing cabinet is should you lose internet service you will need till it is restored before being able to access your online virtual filing cabinet. The workaround for this is to also save a copy on your hard drive.
If these fail, you can always do a search on Google. Start by typing in the model number and manual, instruction or instruction manual. Ex: Omit quotes. I am using a model ABC123 as an example. "ABC123 manual" "ABC123 instruction" "ABC123 instruction manual".
You may find that manual for that old TV that you were looking for in your home and was unable to locate.