Next time we're in Manhattan, we're looking up a place called Psychedelic Solution, which specializes in psychedelic posters from the '60s. It deals in originals and reproductions, art posters and advertisements for rock shows of the era. Cheaper than plane fare, however, is the $2 it asks for a catalog. If it's anything like the solicitation postcard -- a purple and yellow poster for a Jimi Hendrix concert -- it should be well worth it. It's available by writing Psychedelic Solution, 33 W. Eighth St., Second Floor Store, New York, N.Y. 10011.