I bought mine used from a seller on Ebay a few months ago. It's good to find an auction with photos of the actual plate so you can see what condition it's in. Like you said, you probably will need to buy a new state seal if you want it to look complete, as I believe to be sold the real one must be removed.
I managed to luck out and come across one with a pretty good number which I bought
You can see the replica state seal is a little bit smaller than the real one was on mine, but it still looks good.
Sorry I can't help with advice for any other sites to get them from.