Question:

Where can I get the vent tubes and how do I replace them?

Answer: The tubes can be found here

These tubes are pressed in. You should be able to move the tubes back and forth while pulling on them. You can also drill them out, just be careful not to drill too large. Install by tapping them in gently.

In general:

Vents are used so that the float bowl doesn't create a vacuum and siphon gas.

Older carburetors simply vented the fuel bowl to the air. Later, the vent was routed back to the engine so that the gas fumes were burned (pollution control).

For the type that vented back to the engine, it can be left open with nothing connected. If gas comes out of the vent then you are flooding (too much gas) On some models the vent is opened and closed via a rubber stopper at the vent hole and it is opened and closed via the accelerator linkage. This type should work as designed.