The best advice I can give you is to google 'free font download' and try out a few sites, I use some for planning tattoos and I am always guaranteed to find something amazing.
They have previews of everything so once you find one you like, just download it and follow the instructions (usually 'unzip' the files and transfer them into your font folder, you can find instructions on how to do this by googling) and then you can either print out some samples for your invitations or tell the person doing them up which font you like and which site you got it from, and they can do the rest for you.

Hope that made sense!!