Um..the ones you put on your trike.
Seriously, most armrests do the job pretty equally. You don't specify your model of trike, but on the Goldwing 1500 most attach to the radio brackets on the trunk. Same with most on the 1800, though some attach beside the seat at the handles. Most are just a bar (straight or curved depending on maker/design) that holds a pad for the arms. Kuryakyn makes a set for the 1800 with a swing under cupholder. Hondaline mades a set that are just extensions of the speaker top to rest the elbows. Variations on theme.
Note they are arm rests, not designed as grab handles or supports while getting on or off. Too much pressure and things may break.
Just put "(bike model) passenger armrests" into a search engine and look for the best deal on the style you like.
Then you can go for the accessories - pouches, cupholders, covers, etc.