Have you signed up for the mailing list on the Whipper Snapper site,
www.whippersnapperdesigns.com? IIRC, sometimes they have special offers, like reduced/free shipping if you spend a certain amount, or a free stamp with your order.
Otherwise,
www.addictedtorubberstamps.com carries a lot of Whipper Snapper (although not the ones I want right now) and they have a different sale each week. If you can wait it out, maybe you'll get lucky and the WS stamps will be part of a sale.
Hope that helps a little!