If you want plain colors, just go to Walmart or Kmart. They have them all. If you want patterns, I would try making them yourself from whatever fabric can be found at any local fabric store. Do not do the custom-made thing. It's never what you think
There are several places to purchase good quality curtains such as; Target, Pier 1, Bed Bath and Beyond, Urban Outfitters, Anthropology, Tj Max, Macy's, Pottery Barn or any other department store in your area.
There are many different places to find ready made curtains. I would begin by trying a department store such as Macy's or JCPenney's. They normally have higher quality curtains than you will find at a smaller department store.
If you can afford to spend the money JCPenny has some really good curtains. They are thick and they are really nice. If you can't afford the higher price tags Target actually has really nice curtains for a fraction of the price. Often times they even have samples on display.