@ Diego:
Click 'shares' on the left hand menu, then 'view share market'.
Corps with the lowest P/E will generally have demand but no supply - but you can still put in a request to buy them, it's just like ordering products, you have to wait until some more come onto the market.
Look at higher P/E values to find corps with shares that are offered but not bought yet. On FB right now, you have to go down to P/E = 52 to find shares lying around, the exact number will be different in different worlds. Be wary of corps where the P/E is trending strongly upwards of course!