There are several ways to watch Lost in Space legally online or via download either free with advertising (Hulu) or for a small fee per episode (ITUNES, Amazon).
Many Irwin Allen shows are available on Hulu.com, but unfortunately at the present time these are only available to US viewers. Hulu also reportedly have plans to start charging for online viewing in the future.
The search box at the top of the Hulu window allows you to search for other episodes and Irwin Allen shows. When the video is played, an icon appears at the top right corner which takes the video to full screen when selected (Press ESC key to return to normal browser view).
Note, if you live outside the US, the Hulu video will not play.
If you live in the US, episodes can be purchased and downloaded from Amazon.com. Click on the image link below for more details.
The good news is that Lost in Space is available for download outside the US via ITUNES. Check your local ITUNES set up to see if Lost in Space is available in your country.
In the UK, individual episodes currently cost £1.89. You will need to upgrade to the latest version of ITUNES to download and view these episodes.