I've been trying out Game Crafter but they are not cheap and I've some trouble paying a customs charge when posting to the UK.
https://www.thegamecrafter.com/ They provide templates and once you get your designs exactly right to prevent "ink bleed" their web based uploader is better than an FTP system.
You can sell through them as well and do bulk orders for yourself.
I haven't started selling my geomorphs through them yet - as I've yet to design a "tuck box" - which is compulsory apparently - also I'm worried about the random extra cost hazards of international shipping.
-The best part was the rounded corners, but as suggested above, maybe just buying a round-corner cutter is an option. ;)
If it's standard shaped cards - like poker decks I don't know if http://www.drivethrucards.com/ offer a service for people who don't want to set up a shop. I've bypassed them for now because they don't offer square cards.
Almost any photocopy/print shop might be able to give you advice - but I bet it's pricey when they realise that you're not talking about business cards. ;)