I thought you were e-talking about the symbols Iroh drew in the sand, because I was wondering where I could see the "real-life" symbols they are based on. To respond to your question, there's nothing, other than the availability of the corresponding element, that seems to limit a bender. An airbender couldn't bring air with him into a confined space sealed off from air sources, and earthbenders couldn't materialize silicates and phosphates, et al, and can't bend refined metals.