Yes, you can buy black calla lilies lots of places. In fact, there are several different varieties of the 'black' lilies. They vary in shades from deep purple, to chocolate, to nearly pure black.
I warn you, though, that these bulbs are NOT hardy and depending on the variety, can be quite expensive! But, WOW!
What an incredible statement they make.
Some varieties are:
Captain Palermo (nearly TRUE BLACK)
Naomi Campbell
Black Forest
Hot Chocolate
Edge of Night
Here are a few links to buy some:
http://www.callalilyshop.pacificcallas.com/blackcallalilybulbs.htm
http://tinyurl.com/y3bk9s
http://tinyurl.com/y2fedr
As far as preserving them goes, there are several methods.
Here's a link to some of them. I'd pick a couple and then google those methods for more detailed instructions. I've seen callas preserved using the wax method and they're really gorgeous.
Never done it myself, though.
http://tinyurl.com/y63q9r
Happy Gardening!
Polly
p.s. Noticed you added info that you want them for your wedding.
Yes, you can buy them at specialty florists, and yes, they are VERY expensive. You might try buying the bulbs and planting them in hopes of growing your own and preserving them yourself for your bouquet. You can grow callas indoors, even. As I said, they're not hardy, meaning they don't overwinter or even do that well in some areas. Do a little research, maybe try to grow some of your own. It would be another of your own personal touches for your special day. Good Luck!