I found lubriplate in a small 1.75 ounce toothpaste tube style container at the hardware store. Very convienient. A couple of tiny drops, shoot the weekend, and wipe it off before I put the gun in the safe. Seems to do the job fine.
I bought 3 tubes at least 5 years ago, I still have not opened two of them.
I do not think the type of lube on a break open gun is all that critical. As long as there is some lube there, and it is kept clean, the gun is protected. I have used a drop of motor oil off the end of the cars dipstick before in a pinch. Sounds crude, but it is better than no lube at all, especially on a stiff gun.