I am basing this on my Blackberry curve - so if you have another model, it may be different.
The memory card is just a plain old ordinary "micro" card. They are easy to find - I think I got mine at my local Target or Best Buy. Just look in the section where they have the SD cards. The card is teensy - so be careful when you handle it. You have to take the back off your BB & flip up a metal holder - it has a teensy hinge on it. Slide the card into the metal holder & flip it back down until it clicks in place & then replace the back of your BB. Easy peasy.
Once the card is in, your BB will recognize it automatically.
As with all electronics, the best place to buy a new card is online. There are a ton of e-tailers whose prices blow the doors off of anything offered up by Target, Costco, Best Buy, etc. Check Amazon, Tiger Direct, Pricegrabber.com, Pricewatch.com, DigiBuy.com, BuyDig.com, etc.
One word of caution: Some devices are limited as to the size of card that they can recognize. Check the documentation for your model and be sure of what you're buying.
I ordered my card with my Blackberry Pearl, probably payed a lot more than I needed to, but was told by the Alltell rep I NEEDED it. I hardly have anything on it! I don't use mine for anything nursing related though.