The required tandem style breaker is readily available everywhere. Different brands are interchangeable. For example, our WFCO has factory Square D and Seimens breakers that I added. The face of a breaker has the series/type of breaker it is - HOMT, Q, QO, etc. The amp rating for a tandem breaker would be "3020". If you aren't sure what to order strictly by the correct numbers & letters only, the easiest thing to do is take it to a store and get them to match it. Or go to an electrical wholesaler. You *could* take a photo with your phone but the lettering is teeny weeny and barely readable to the naked eye. WFCO should have a list of breakers that can be used on their website.
You don't have to use a tandem breaker as long as you have enough space in the panel. You could use a 20 amp breaker and a 30 amp breaker. The 30 amp breaker is the main breaker and the 20 amp is usually for the AC unit.
These are sometimes called a tandem and sometimes a duplex breaker. Note well that you want a tandem breaker that is single pole 120 volt and not a 2-pole 120/240 volt one (with tie bar) like you'd find in a house.
Out of curiousity, why are you needing a new 3020 amp breaker?
Below is a Square D "HOMT3020" and Seimens "Q3020" tandem 30/20 amp breaker.