'I'm getting ready for work.' We say this when we're getting ready at home in order to leave the house and go to work. Work here is viewed as an event, hence, the use of 'for.' It's equivalent to 'I'm getting ready to go to work.'
'I'm getting ready to work' is also a valid phrase, but it's more general. Here, work is more like an activity. If someone works from home, they may say this instead of using 'for,.' It's more or less an equivalent to 'I'm getting ready to do work.'