There are some farms that have work share programs, often as an alternative form of payment for receiving a weekly food box, though not many advertise, so it's hard to know where to ask. I think, from a farmer's perspective, it's sometimes hard to gauge whether or not having a large work share program is worthwhile, since it often requires so much planning and day-of management. I know Sol to Seed in Carnation has a work share group at the farm every Saturday and others may have save similar schedules. I think your best bet is to visit one of the bigger markets (U-District, Ballard, etc) where the farmers tend to come themselves (rather then sending a sales person) and just ask them directly.