I have a ryobi drum sander and the drum started vibrating, so I opened it up to clean out the dust in the drum and found three pcs. of lead
loose in the drum that must have been used to balance the drum. Problem is , I can't see any signs of where they were attached. Have any of you had this problem or does anyone know how I
can balance the drum?

have you priced a new drum?

I've checked out the internet for parts and they
say a drum is no longer available.

a machine shop might should able to balance the drum for you

You could put marks around the drum and keep moving the weights around with double faced tape till it runs smooth, then epoxy the weights at that point.

http://www.ereplacementparts.com/ryobi-wds1600-wide-drum-sander-parts-c-7931_7932_161325.html

Snoop around in here if you haven't already. Price on drum seems high tho.

Oh, and check the bushings/bearings on the drum too. I've had them make vibration before.

Where were the weights on the drum? Was the drum enclosed?

You should be able to have it re-balanced for less than a new drum. I'd check the bearings first like Kenny said, then start calling around. Depending on where you live, you might find a place that just specializes in balancing. Or you can send it out -- look online. Otherwise there should be a machine shop locally that can help you out too.