I can't speak to having a knee injury, but I can recommend med balls. I got a set of these at Dick's Sporting Goods. They are often Buy One Get 2nd for 50% off, but 25% each is comparable. The grips on them are excellent. I got 4 thru 12lbs, but they don't make the 4lb ones anymore. They smelled at first, but were fine after wiping down with soapy water.

If from Amazon, these are the ones Tony Horton has in his own home gym. I'd say that's a pretty good endorsement.