There are several ways to watch Lost in Space legally online or via download through various streaming apps including Hulu, ITUNES, and Amazon.
Many Irwin Allen shows are available on Hulu.com, but unfortunately at the present time these are only available to US viewers.
If you live in the US, Lost in Space episodes can be purchased and downloaded from Amazon.com. Click on the image link below for more details (#ad).
Lost in Space is also available for rent or purchase on Amazon.co.uk (#ad).
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.