Homemade Sausage Making

I recieved a meat grinder/sausage maker for my Kitchenaid mixer. I'm really excited to get started in making sausage's. Where on line is the best place to buy casings? What size should i buy for my tubes? one is 3/8" - 1 cm and the other one is 5/8" - 1.6 cm.
Thanks for you help

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