I've not been able to find just the rubber boot. You'll have to replace the tierod end to replace the boot. I have an old bicycle inner tube that I'm going to put on my BX if I ever get a round tuit.
One of the front steering boots is slightly torn on my BX2350. How big of a deal is this? It doesn't look as if its filled with grease or anything. It looks like it would be difficult to replace it.
I just replaced a boot on my L3940 it took about a half hour. Just unbolted the tie rod and unscrewed it, counting the turns, slipped the new boot on and screwed the tie rod back in and bolted it up. The part was $17.00 at the local dealer.