Generally approaching people at a store or convention hall with "Hey, do you have trades?", "Hey, can I take a look at your binder?", or if you're looking for a specific card "Hey, do you have [specific card] for trade?" should be enough to get started
And you have a few Zombies you can toss in as a few adjuncts to your strategy like Gray Merchant of Asphodel, Cackling Fiend, Fleshbag Marauder, Sidisi, Undead Vizier, and Custodi Lich, all great Zombies with strong abilities for multiplayer already built in, without really committing to a Zombie theme
Its worth mentioning The Lost Cavern of Ixalans Explorers of the Deep Commander Precon, which cares quite about Merfolk and +1/+1 counters but if you arent fussed about budget, Universes Beyond: Warhammer 40,000s Necron Dynasties is an incredibly strong Commander Precon, especially if youre a fan of the Games Workshop franchise (or Space Marine 2)
The original marketing posited players as the planeswalkers, with their deckscalled librariesserving as a compendium of spells theyd learned and fearsome creatures they could summon