Twenty years ago, I knew someone who made a living doing greeting cards for (I think) Blue Mountain. From their web site, it looks like they mostly do online cards today, though, so they may have gotten out of the print business.
I'd check a recent copy of Writer's Market at your local library - they no doubt list greeting card companies. You can find their website
here. Other potential sources for greeting card publisher names & addresses are
Writer's Digest and
The Writer Magazine.
Vasily