You’ll really stifle to source a genuine Omega crystal yourself. The parts houses like cousins can no longer get parts from Omega.
Your chooses are;
1. Go to an Omega certified watchmaker like Swiss Time Services who can install new as part of a service.
2. Try your luck on eBay or similar but it might take months or longer.
3. Get a generic crystal from somewhere like Cousins.
4, Finally, confirm you actually need a new one. If it’s cracked, yea a new item is required. However, if it’s just scratched you can usually polish them up really well to like new with polywatch in ten minutes.