WHERE CAN I FIND ___ ON VIDEO/DVD IN ___ FORMAT?

Take a look at the Where to Buy Rik Items  part of the website -- plenty of links to all sorts of stores are there.

If you just want a quick survey of what's in print right now, go to amazon.co.uk, amazon.com or play.com and do some searching. If something is in print, it'll be listed on Amazon for sure.

You can usually find out of print stuff on Ebay pretty easily. Check both Ebay.com and Ebay.co.uk (there are other Ebays as well, but you'll find the most Rik related stuff on these two). Be patient with Ebay and what you want will usually pop up eventually.