Looking for a backpack I can weight and run stairs.

Sorry if this doesn't really belong here, but I thought it was related. I starting training for firefighting and was looking for some way to add weight to my workouts without running with weights in hand. I figured camping/hiking/climbing bags were the way to go due to the chest and belly straps. Would you guys have any suggestions for bags that could hold lets say 50lbs and be under $100?

thanks