I have more circuits that I can add onto my service panel but have run out of neutral slots on the bus-bar what can I do?

Absolutely. Every manufacturer will offer a neutral bar kit for their panel. What you need to take note of is where is the Main Breaker located. If in the panel that you will add the bar to, then the neutral must be bonded to the metal can of the panel. This usually is done with a long green screw that is part of the kit, drives through the bar and screws into the metal of the panel.

All white neutrals and bare or green equipment ground can connect to this bar.

If the Main Breaker is remote, then the neutral bar kit will be insulated metal box by the mounting feet of the bar, and the Main Bonding Jumper, (green screw) must not be installed. Only white neutrals will connect to the neutral bar, equipment grounds will connect to another bar that is connected to the metal box.