If you are no longer motivated to work in your current, would it be best to quit straight away or stick it out till you find an another job?

it's better to find a replacement before quitting, definitely.

I wouldn't recommend giving up a job until you have found another first. It looks bad on a resume. And one can never tell how long if one quits a job, it will take to get another. Financially it is a mistake too. Try to find some way to cope with the unsatisfying one until you get a better one.

I would always give your current employers notice, but probably wise to line something else up in the mean time.

Best to look at other options first before quitting...