I want to add flaps or flaperons to my Nexstar trainer to slow it down a bit for landing in an old field by my house. I have an 8 channel transmitter and receiver but I need one or two additional servos. I have two Tower Hobbies System 2000 TS-51 standard servos. I believe the TS-51 is for RC cars and trucks. Does it matter? If so, what's the difference?

That's already a slow landing plane. I doubt flaps would make much difference aside from helping it slow a little faster.

Agree with Jester S1.
However if you really want to slow it down whatever amount it is, you can make slotted flaps (like fowler flaps without the backwards movement to keep it simple) taking up to 30% of the wing (or more if you use flaperon mixing because you will be flying so slow the ailerons are not enough to control bank).
Leading edge flaps or slats also give a great help, however they limit the max speed quite significantly. I think the slotted flaps will get you what you are looking for.

Fly safe, be safe.

The decision really comes down to what you actually want to accomplish. If you find that the plane is floating past you and just won't come down, flaps will add some drag to help it bleed off airspeed. But they won't actually lower your landing speed all that much. I refurbished a Hangar 9 Cub a few years ago and decided to add flaps to make it a Super Cub. I found that actual landing speed was about the same (still slow) but the plane was more draggy. Others have commented that the Avistar Elite's flaps have about the same effect. So flaps will shorten your landing approaches, might shorten your takeoffs, but they aren't going to make your plane easier to land.
What they will do for a beginner is cause stalls to develop sooner and get worse faster. I would never recommend a beginning pilot fly a plane with flaps for that reason- the plane becomes less forgiving of bad speed management or a bad approach on landings.
I'll also ask you to rethink your goal of slowing the landings. The more slowly the plane lands, the more susceptible to wind gusts and turbulence it becomes. Again, in my Cub example, there are some days that I really have to be on the sticks while landing because it reacts to every little disturbance. Compare that to my Sweetater pattern plane that has probably double the landing speed (still not overly fast) and it's a joy to land.
So what's actually the issue? Are you finding it hard to flare for landing as the plane dives for the runway? That's a symptom of a nose heavy plane. Or is it just that you have control but find things happening too fast for you to keep up? That's just a matter of experience.