Just search on ebay. I bought both a 512 and a 1gb on there well below retail. Make sure it's a SanDisk brand, and you shouldn't have any problems. I paid immediately through PayPal, and I had it within 3 days.

I always check out the seller's feedback to make sure it's positive. I order a lot of things off ebay, and have had very few issues.

www.newegg.com

I highly recommend www.deals2buy.com it is a site that daily compiles good deals on the internet. If you search for "miniSD" on that site you will see the current good deals on various memory levels of miniSD cards.
-K