I bought one-day, one-park printout tickets online last week. It worked like a charm -- no problems or holdups at all exchanging them at the turnstile on Friday.
It takes a little clicking around at disneyland.com to find a place where you can put one-day, one-park tickets in your cart, but it is possible. From this link:
http://disneyland.disney.go.com/disneyland/en_US/reserve/ticketListing?name=TicketListingPage
Click on any of the park hoppers on the tickets page, then select "All Tickets" from the dropdown menu (click "Continue" if it asks you to resend), then click on "Theme Park Tickets." You should reach a page where you can scroll down to a "1-day, 1-park" option that you can add to your cart and generate printouts.