Google: use this search operator, stocks:, with your query immediately after the colon. Unfortunately, you have to know the stock symbol to do this little trick; Google won't find information based on company name.
Yahoo: Yahoo doesn't have a specific search operator for searching stocks, all you need to do is type the ticker symbol into the main Yahoo search bar and you're good. The best way to find stock information at Yahoo is Yahoo Finance; you can look up information using ticker symbols, company names, security/index name.
Ask: Now, Ask is a bit different. I didn't find a specific search operator here, but a specific search query: "quotation for nike stock price". If you just enter the word "stocks", you might be able to find a few stock information quotes, but their index doesn't seem to be very exhaustive (Yahoo, Enron, and Microsoft were missing from the stock information). Overall, I think Ask needs some work on this particular feature.