C.W.
When we converted our daughter's crib to the toddler bed I wanted to buy a rail for this reason, too. When shopping I found that all the guards and rails could only be used with a regular mattress and boxspring and were not recommended for convertable cribs. I think there is an inflatable bedrail out there but I am not sure if it is short enough for a convertable crib. Our daughter did fall out of the bed one time and that was it. It scared her more than anything.
Shortly after we converted her crib it was recalled so we ended up going to Walmart and buying one of their toddler beds. It has little rails and she never had a problem with falling out.