Talk to your homeowners insurance agent. I did and this what was explained to me:
If your collection is in your home, it is covered as part of the contents of your home as a part of your standard homeowners insurance policy. If it is kept outside your home or likely to be damaged or lost by something not part of your home, get a separate coverage rider.
Think about the risk to your collection. Where do you keep it? Is it in a basement that is prone to flooding? Is it kept under pipes that are prone to breaking? How could it be damaged? It is most likely to be damaged as part of some other disaster like fire or flood, so make sure your home (and contents) are included in that risk. Flood insurance might not be available to you and sewage backup insurance is hard to come by. Know if your basement is below the water table and do not keep valuable items (collection or otherwise) on the floor if it is.
Biggest thing you can do for yourself is to inventory everything you have. Hello, that is what this site is for! Put all your cards that you own into this site and maintain an accurate inventory. Take some pictures of your collection, especially the most expensive ones, in your home to show proof of ownership.
If you rent, it is completely different. You will want to talk with your insurance agent again, but your renters insurance covers only contents. Again, inventory your collection and take pictures, but you will want to have a more accurate appraisal of the value.
Remember to discuss replacement value of not only your collection, but all of your insurable items. Some things depreciate in value over time (autos, standard furniture-not antiques, clothing, etc). Your cards, money or jewelry likely will appreciate over time. We have a rider on my wife's jewelry because it is more likely to be damaged or lost outside our home where our homeowners policy would not cover it.
Bottom line, everyone's situation is different, so speak to your insurance company. Inventory your entire collection and take pictures.