There is one way to evolve Trade-evos, with an item called a Linking Cable. This item costs 1000 MP (Merit Pts.) and can be found at the Jubilife Trading Post. As how to acquire MP, all you have to do is scourge the Hisui region in search of various lost Satchels. They can be located via the map.
Also, most trade-evos' final forms can be found in the wild.
TLDR: You get MP from treasure hunting other people's lost satchels to then purchase the Linking Cord OR you can just catch the final evo of the trade-evo (I.E. Alakazam in the wild.).
Source: Knowledge of older leaks and experience to deduce
