It may be possible to get a 'rider' to your renter's insurance policy for the collection, rather that separately insuring the collection. But you're going to have to shop around.
If you want to know the rough market value of your collectables, you can find an appraiser and (pay to) have your collection appraised. That could help you establish your insurance needs and should help document the contents and value of the collection to the insurance company in the event of a claim. If you make a claim, your insurance company will use its own metrics and experts to try to determine value.

