We can’t give you shopping advice, it will depend on your location through which channels you can obtain a replacement part.
What we can help you with is to determine what size of pot you have - and based on that, you can pick the correct size.
- First, you can measure the diameter of the “no longer there” gasket, some web shops may actually list that value.
- If they simply sell by pot size, measure the amount of water that goes into the lower compartment. For my Bialetti models, the three-cup one takes about 140-150 ml, the six-cup around 300-310 ml. That translates to about 50 ml per cup (will be less after brewing, as some water will be absorbed by the grounds).