Just make sure any fake plants you consider are color-fast and won't bleed any dyes if they get wet. That's more of a concern for aquatic turtles, but may be a consideration for tortoises as well. For reptile-friendly plants, LLLReptile has a really good selection.
I use live plants, but in the past for other herps, I've found the most realistic looking ones at craft stores like Michaels and Hobby Lobby. If you have any shops around you that specialize in artificial plants for decor, you might check there. From what I understand, the fake ones need to be plastic and not the silk/cloth type. As Tom said, they can try to eat them, so one needs to be concerned with possible blockages.