They're fairly standard residential circuit breakers. According to the 8900 series manual from WFCO, the following brands and types are acceptable for main breakers:
Cutler Hammer: BR and C
Thomas Betts: TB or TBBD
ITE/Simens: QP or QT
Square D: HOM or HOMT
Murray: MP-T or MH-T
General Electric: THQL
In addition, for non-main breakers, Cutler Hammer BRD, BD, and A types are approved.
You may find you need to order a 30/20 combination breaker as that is not a commonly used combo breaker anywhere other than in RVs. An actual electrical supply house would be more likely to carry it than a home improvement store. If you have an empty space, or a single breaker for some other branch, it may be worthwhile to rejigger things so the main breaker is a non-duplex breaker.
