As has been stated earlier you can find heavy discs. Look at some old lightning roller discs, those things are normally 200g+ and they are cheap.
One thing that was alluded to was warming up your shoulder. Look up the 7 min. rotator cuff solution exercises, I started doing this years ago and trust me it helps. You can easily do it with a few discs in your hand and they are simple arm motion exercises. I normally use a 5lb dumbbell to do these when working out at the gym and trust me it saves my shoulder. This also helps develop the tendon and ligaments in your elbow which will help you in the long term if you do multiple sets of these.
If your trying to work on strength, I think the best thing you could get are those forearm exercisers that you squeeze. It will increase your hand and forearm strength considerably and probably help you develop more snap over time.
Being big and strong doesn't mean your gonna be able to throw far, I'm a living example of that, the furthest I could drive is 400ft on a terrific day and I'm built like a linebacker. My buddy that's more than half my size and is a bean pole huk's drives much much further than me and he couldn't bench press 175lbs if his life depended on it.
It's really all about form and snap to get good distance