I have received many requests for replacement keys for Sentinel gun cabinets. I guess I rank well in Google for the post I did about Sentinel gun cabinets or maybe Sentinel gun safes. Unfortunately, I am not the manufacturer or a dealer of these products. In fact the Sentinel product line was a private label brand manufactured by StackOn Safes and sold in big box stores like Walmart. I have heard that some of these store may still even carry them.
They were a pretty good product coming in either an 8 or 10 gun configuration, but were mostly built like a heavy metal locker more than a safe. They can be secured to the floor, but could be opened pretty easily with a brute force attack of a fire ax or sledge hammer. Ultimately, at the low cost, they were good at securing your guns against children and less determined thieves. Stack-on makes newer models now that start at 8 guns and go up from there.
As for keys for these older units. You should be able to order them from Stack-On. They have an order form here. While I have not done this, I assume they can reproduce your key with out any issue since they produced these Sentinel gun cabinets. They say the cost is about $10.
Alternatively, most locksmiths should not have any trouble getting this unit open and making a new key for it. They can use a pick to open it and then remove the lock inside to decode the key cut. Might cost you $20 plus the keys. If you have 8 long guns in this plus ammo, I suspect it may be pretty heavy. Some old-school locksmiths will still make housecalls. Might be worth the $50 trip charge not to have to figure out how to transport this gun cabinet fully loaded!
Last option is to get a crowbar and open it manually. Obviously you will be damaging the unit. Maybe you could bend everything back into place enough so it works, but it will be compromised for the next attempt (which may be a bad guy). Heck, maybe this is a reason to buy a new Stack-On GC-14P 14-Gun Steel Security Cabinet!