Mr Roo: WELCOME to iRV2.com
We are glad you joined and posted your first question. The air dam is a Workhorse provided part, and could be ordered from any WCC dealer. You may also want to look at home improvement type stores for a heavy duty mat-type material you can cut to fit. Use "fender" washers when reattaching.
Whatever you decide, the air dam NEEDS to be in place to ensure proper cooling and less cycling of the fan clutch which makes the loud fan roar you are most likely hearing too often.
You may also want to try our "find" feature. It is a search engine which will turn up more info from previous postings.
Thanks again for joining us! ED