I did a quick Google and found that there are lots of wedding-themed websites which offer personalized magnets for this sort of thing. However, they seem to charge quite a bit -- typically $10-$15 per 10 magnets, plus shipping. Some offer pre-designed, generic "save the date" magnets that are a bit cheaper, but they look pretty tacky to me.
If it were me, I'd use Cafepress.com. That way you could design the 2x3 rectangular magnet yourself (they provide the template) and can get 100 magnets for $85 plus $5 for flat shipping. They also have 2.25" round magnets, $75/100. (You don't say how many magnets you'll need, though.)
I have designed and bought several magnets through Cafepress and they are very good quality. They're not wimpy magnets but the sturdy kind that have a strong magnetic grip. The advantage of this is that they'll hold up for the long term; the drawback is that they may cost you a bit more to mail out to guests.
I have a degree in graphic design as well as the necessary software (Photoshop, etc) so it comes pretty easily to me. If you need some help designing the magnet, let me know. I enjoy doing that kind of thing so it would be free of charge.
~ Jenney