I use bobbers from this site...The Everlasting Slip Bobber - the world's finest slip bobber!
They are quality bobbers and I have never had to buy another bobber since then as these are great and last!
I have green LED lights I use to draw in plankton which draws in bait fish which in turn draws in the predator fish like the crappie.
Buy the size you want based on the amount of weight you will need to use. I use the second to the largest on 6 rods I own and put a 1/4 oz weight on them and they sit about 40% under water and 60% above the water line to see the bobber well. With the green LED's it makes the bobber "stand out" as if it were fluorescent.
Trust me when I say these are the best bobbers out there.
As for your slip knot easily moving, make sure when you tie the slip bobber sting knot to your line that you use pliers to pull it as tight as you can get it on your line. This will keep it from coming loose and sliding on it's own. You can slide it as needed by pinching the knot tight and sliding to the position you need.