Mattress - Neck and shoulders

[Avi92]

I get a cervical Spine strain in  my neck. I used to get it every 3 days a week. And I had to take a muscle relaxant every time. I started walking and doing some exercises  with weights recommended for my condition. Now, I get this pain once or twice a month and that too I don't have to take muscle relaxants . A mere pain reliever helps me out (Although I try to ignore taking it and walk and exercise to get it better).

I sleep on sides, back and front sometimes. I do not have a definite sleep style. I moved into a new place and want to buy a mattress. There are sometimes when i wake up with one of my hands locked. I am really confused as to what type, brand mattress I should buy. My budget is 400 for a preferably queen mattres(i might be able to make full size work too).

Please give me advice. Also i would love to go and get it from the store itself. Online mattresses are also welcome.