I'd say there's about a 50% chance you'll find an equivalent part on Mouser / Digikey / Newark. But it could take some work. Start by looking at the major tactile switch manufacturers sites and look for something that looks the same. Newark have good big pictures of parts. Try Google images. Try Ebay. Chances are it's not going to be exactly the same but if the footprint and stem are the same it shouldn't make a difference. You can also compensate for differences in height a little bit. The stem is going to be the critical bit to match because, even though they may look exactly the same, they're really all different. Even if you think you find something with the same dimensions you won't really know if the old cap is going to sit correctly on the new stem until you actually just buy one and try it. It's got to snap on just so. But you might get lucky. If you find a new part that actually fits the cap perfectly, you might consider replacing all of the switches so that they all have a consistent feel. Otherwise, the one is going to feel a little different probably.
I would also contact Lexicon. They might have replacements. If not, maybe they have a datasheet on the old part or at least critical dimensions.