Any pet store should have a slicker brush for you.
Some groomers may allow owners to stay and watch, but to be honest, all of the places I have worked do not prefer it.

1st, it is disruptive to all the animals when strangers are present.(remember, most of the pets in the shop will be dogs. They tend to want to bark and jump around when people are present and more so if it is their own person). This makes them hard to safely work on and it gets all the other dogs wound up too. Now cats don't really care if their owners are present but can be affected by all the barking.
2nd, most places do not start the pet immediately upon its arrival in the shop. Except for the smallest businesses, animals are worked on in rotation, not straight through. So kitty might not get started at drop off time, so you'd need to wait for the procedure to even begin.
3rd, sometimes even the most gentle handler will need to use a procedure that looks a whole lot worse than it is in reality and owners have been known to misunderstand what they see. For example, when a cat is getting a total shave, in order to get the clipper down the throat area and between the 2 front legs a second person must hold the cat up by the neck. Now don't faint on me, that is what it looks like from a distance, but in reality I have my fingers of one hand on their lower jawbone, not the throat or windpipe, while the other hand hoists them up under their ribcage. But I often joke about how I have to "Hold the cat up by the neck", because that is what it looks like.