Best to get a recommendation from a parent with a child who has a relatively high enough score to secure a place at one of the grammar schools. There are so many tutors advertising services its difficult to assess the good from the bad.
If you are prepared to put in the time & effort -you could tutor yourself. I've never got tutors because of the cost - and still succeeded,
my dc3 has now got a decent score so am hoping she'll get in. For me, its three down one to go- thank goodness I've got a 2 year break till I prepare my youngest!