Can not find staples for 502 ace liner stapler ,

SOURCE: Staple Gun Troubleshooting

Most of the staple gun models lack an "anvil" — the metal plate with curved slots that office staplers use to bend the legs of the staple inwards and flattens them against the paper. Staples set with a staple gun retain their straight legs, and are held in place only by static friction of the legs against the compressed surrounding material, much like common nails. Indeed, some staple gun models can handle brads and nails as well as staples. Some staple guns also use divergent point staples where the legs twist as the staple drives into the surface providing superior holding power.

SOURCE: Stanley TR70 Manual Stapler won't accept Arrow 1/4" staples

It sounds like you have a light duty stapler and are trying to use heavy duty staples. 1/4" refers to the length of the legs not the thickness of the actual staple. You probably just need light duty staples.

SOURCE: refilling the staple gun

The staples do not go on the bar you pull back but directly into the bottom once you have pulled the bar back. I was trying to put the staples on the bar at the back and push them into the stapler.

SOURCE: Swingline Stapler Staples Only After You Open It Up....

When you lift up the top of the stapler (as though you are going to put staples in), does the pusher slide freely back and forth? It might be getting stuck at some point and not pushing the staples (either due to a worn pusher spring, or friction). First (after removing the staples) try spraying some light oil on the mechanism and lifting the top part of the stapler up and down a few times and observe how easily it moves back and forth. Also, with the top open try GENTLY sliding back and forth a little - if it slides fine with no sticking, try loading staples and using stapler. If it slides ok, but doesn't quickly return to it's "normal position" when pulled so as to put tension on the spring, rhen the spring may be worn and need replacing.

SOURCE: Disassembled the stapler now need diagram to reassemble

Found user manual online but no parts breakdown, I have a phone number at 1-800-343-9329 and they might be to email you the breakdown. Wish I had better info but this is all they furnish online.