The lens is glued to the housing. I have seen methods of heating the assembly up in the oven to loosen the glue but I have no idea where you would get just the lens. If you did, getting it resealed without leaks would be very difficult.
Are your existing lenses broken, or are you replacing because they've faded? I had my yellowed/faded lenses restored by a detailer and now they look like new!