How much would it cost to get new/repair my shocks in my 1996 toyota avalon?

[caleb.carl]

I have a 1996 Toyota Avalon and when my 4 kids (300lbs) get in the back the back end goes really low to the ground. It does not look good. I want to get new shocks or repair them. How much would it cost and where could I get it done?? Thanks a million