If you are on a track get one...please. I know that some dont require it, but as a common courtesy to others I think its a good idea. I know this is racing, but at Mid O this year I was in 2nd behind another racer in first. He was blowing smoke like it was going out of style. I dont like being behind guys blowing smoke and was pointing to his bike to the grid marshall as we are going down the front straight. I backed off a little, nervous that he was gonna grease the track, and hitting oil you go down quick, and usually with no warning. They black flagged him and found his counter shaft seal was leaking terribly. In the belly pan was about 1/2 quart of oil, enough for a big mess and lengthy track shutdown for cleanup that was avoided by the belly pan.
On to your question. You can either get a set of race bodywork with a bellypan, or a common use is actually a turkey baster pan bought at the grocery store. Not very neat looking but effective.