You can use PBS or HBSS (without Ca2+, Mg2+). For antibody based fluorometric detection you can supplement the solution with 0.5 -2 % BSA.

My advice is to use phosphate buffer saline .

Cells dont like to be in PBS for long. How long does the magnetic separation last?

I would suggest that once the cells have been lifted from the dish, use their culture medium as the suspension medium. You may need to look at what the bead vendor recommends for proper binding.