If you want to sell your art on greeting cards,Try licensing them to card manufacturers.
I have managed to get a contract with a card company.Who have taken on several of my images.They are starting to sell quite well.And I have received several royalty cheques.
It is also a great way to promote your art,As the artists name is printed on the back of the cards.
If you live in the UK. I find the Greetings Card Association website is very useful.It has pages of companies, who are looking for submissions.
I have only ever sold 2 cards on here on Redbubble.