Madison Holistic is definitely a legit agency... Though I don't know Sandy / Gretta personally so I couldn't help you out there.  I know Madison Holistic does or did have some reviews on this website, but unfortunately I do not have a link right now.

I can definitely recommend two ladies I know in town for sensual BDSM.  One is Penelope Waters (FBSM/BDSM) and the other lady is Lena Duvall (escort/BDSM).  Both are personal friends of mine and would be excellent choices!  
http://penelopewaters.net/
http://lenaduvall.com