I would absolutely look locally, ask a policeman where they would recommend you shop or google gun dealers in your area and then check the reviews.

Under "no" circumstance would I suggest you do anything with GunBroker except delete, block and ignore. Run form them.
Integrity does not exist here.

My guns were withdrawn within a week or so of listing and now almost a year later they are still trying to access funds from my credit card. These people are not to be trusted with your personal financial information.
Unless of course you enjoy drama, problems and working with people of questionable ethics.

Dont bother to look here you will get screwed like so many others

You have to look for a clip for the Make and Model of the firearm

I am not a modern gun dealer, and am not familiar with what firearms take what clips. You do not provide enough info anyway. I advise contacting a licensed dealer in your area and either take the firearm to them to identify what clip it would take, or at least copy down any identifying model numbers and the manufacturer's name so they can work from that.