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I think you can go with something common and cheap and easy to keep indoors - like a Pothos plant or something. It's non-toxic but I doubt your T will bother with it at all anyway
I really wanted to do live for my G. rosea, but they are not supposed to have much humidity, and the plant would require watering.
Unless you can find a spineless succulent (maybe aloe vera?) you should probably stick to silks.
Was going to say silks or plastics maybe your best bet but they are expensive (At least near me they are) I have found tho cake decorating shops or big brand shops Boyes or Wilkinsons for example sell fake plants and they tend to be cheaper.
I'm not saying it can't be done, but unless your very good with plants it probably shouldn't. Grass in a rose enclosure is far from ideal, as the grass and a rose have different requirements. A lot of plants can create Toxins that are very harmful to Ts and a lot of plants have been sprayed with mild pesticides which are obviously harmful to a T. With silk you won't have thos problems.