Are all 9 speed chains compatible with all 9 speed systems?
AFAIK any 9speed chain made for dérailleur should fit (single speed and hub gear chains are different)

9 speed chains are more expensive, they are thinner and not as common - so fewer cheap brands bother to make them, or stores to stock them

BUT - the chain is an important part of the bike, a poor quality chain will stick, jump and wear the rear cassette teeth more quickly. You might not want to buy the very cheapest brand you can find.