the web site will not show you another receipt. It will, however, show the conf # in the upcoming travel itineraries screen if you were signed in with a RR # when you booked it.

If you call Southwest and can verify the credit card (or gift card) used, they will re-email or re-fax.

Other ways of doing this:
1) at the airport, when you get to the auto check-in computer, one of the options besides reprint boarding pass is reprint fare/itinerary receipt.

2) you can trick the website into showing you how much $ was paid (but still not a full receipt). do this either by: pretending to cancel: the last screen will tell you how much $ would be refundable/refunded, right before clicking on 'I agree, cancel now'. or a safer way is to click on change itinerary, and pick any random new flight or even the same route. Before hitting OK to change it or paying them $$, it will prompt you to pay for any fare difference, and that screen shows the $$ originally spent on the route.

3) if the fare hasn't changed, you could always cancel & rebook.