Um ... authorized dealer? But seriously, just wait until it is time for a full service and have it done by the service center. They will replace the crown.
Finding one the right size will be the challenge - I measured it as 6.7mm diameter x 2.3mm depth (no idea of the thread size) and can't find that size on any regular online dealers (Cousins, Ofrei etc).
... but I'm with Dan S above; if you take it into a Tag Heuer boutique then I'm sure you'll be able to get them to replace it. Given that the CS3111 is quite likely 20 years old now, maybe it's due a service anyway 😀
I was playing with mine the other night, trying to figure out which combo of strap vs Abel Court BoR works best...