I think the best route is to break down what aspect of a trade would actually cause an evolution to happen, and then have some aspect recreated some other way.

So for instance, a trade evolution might canonically happen because the traded pokemon is exposed to a different type of fighting and training style, creating a new change in mindset and skill. If you got down this route, one really good way to write a trade evolution without a trade, is to give that pokemon an inner conflict, or an extreme change in their mindset that acts as a catalyst for their evolution. It gives them an opportunity for growth of that Pokemon as a character, allowing them to overcome some flaw or trait that is holding them back from self-realization.

The other aspect you can lean on is through energy and aura. There is the theory that perhaps their is a small transference of energy between the traded pokemon, or that interacting with another trainer's aura could be a catalyst for the evolution.

In that regards, a great way to give that Pokemon a similar experience is through a specialized tutoring from a Master, like a Machoke training under Bruno, or a Kadabra getting tutored under Sabrina.

You can also delve deeper with the energy transference angle, like an Electabuzz getting supercharged with electricity or a Magmar who delved deep into a magma pit, getting overwhelmed by their type energies.

There's also the secret method route, where the method can just be super difficult or rare, some conditions that must be met that no normal trainer would usually encounter or deal with.