Well mine are definitely not providing a seal on the drivers side, and during a horrific rain yesterday let water through to a coil.

I see for $26 I can get a new Coil cover, but they're in great shape and it's just the gasket. Is there anywhere that just sells the gasket?

 

I haven't seen the gasket alone. If you can't find it on ebay it probably isn't available.
If the gasket isn't damaged you can use some grease to seal it. Some even use hi-temp RTV.