Trade evolutions

You never need a second player. Every Pokemon Center in all four regions has a trade machine: a scientist standing to the right of the nurse's counter. Talk to him, choose a Pokemon, and it is sent out and back as if it were traded, so a Pokemon that evolves by trade evolves on the spot.

If the evolution needs a held item, give the item to the Pokemon first. The item is used up, as in a real trade.
A Pokemon holding an Everstone does not evolve.
If nothing would happen, the machine says so and the Pokemon stays as it is.

Every trade evolution

Pokemon	Evolves into	Held item (used up)
Poliwhirl	Politoed	King's Rock
Kadabra	Alakazam	none
Machoke	Machamp	none
Graveler	Golem	none
Slowpoke	Slowking	King's Rock
Haunter	Gengar	none
Onix	Steelix	Metal Coat
Rhydon	Rhyperior	Protector
Seadra	Kingdra	Dragon Scale
Scyther	Scizor	Metal Coat
Electabuzz	Electivire	Electirizer
Magmar	Magmortar	Magmarizer
Porygon	Porygon2	Up-Grade
Porygon2	Porygon-Z	Dubious Disc
Dusclops	Dusknoir	Reaper Cloth
Clamperl	Huntail	Deepseatooth
Clamperl	Gorebyss	Deepseascale

Read from the game's evolution data: 17 trade evolutions.