Are you talking about the metal cage that goes in and has all the tabs to bend and keep it in place? I don't know if they're standard across all manufacturers or not, but I know I found one pretty cheap on eBay when I went to put my old Pioneer HU into my parents RV.
I seem to recall the Pioneer being more than that from Pioneer direct, but its been awhile, so who knows. Maybe I went eBay just because I knew I could get it quicker (Same state seller). But who knows.