One of my plastic lids has a small crack/leak :-(
I heard kegconnection.com might have them (used) but their website is AWOL right now.
Cornykeg.com also sells lids. I haven't bought them, but I have bought several of their pin lock to ball lock conversion kits and some gaskets/o-rings. No complaints with their service on that order.
My father says, "hobbies are supposed to cost money".