Depending on where you live, your best bet might be a health food store; that's where I used to buy them (Northern California). I have also purchased them at a "feed and seed" outlet (central Ohio) that catered to farmers, since alfalfa is a common hay crop.

If not by using a web search engine, I'd start with your Yellow Pages under "health foods."  But if you live in a rural agricultural area, head to your farmer supply depot.

Here's a web page that includes both instructions for growing alfalfa sprouts and (in the right hand column) links to seeds for sale on Amazon.