Where to buy lead (or other) weights for HO locos?

[RobertInOntario]

Any estimate of what the maximum weight would be for a typical HO loco? Rob

Rob, this will depend on the individual locomotive. When I was attempting to improve the pulling power of my Athearn Mikados, I made an "experimental" weight using sheet lead. Forming it into a "saddle-shape", I draped it over the boiler, held the tender with one hand, then applied power. The drivers slipped easily, showing that the applied weight was well within the limits of the motor's capabilities. Unfortunately, there was no way to physically cram all of that extra 22 ounces into the loco, as it would have been a great puller.
A simple way to test for the maximum allowable weight that can be added is to perform a similar test on each loco - for most, it will be impossible to hide that weight inside the body shell. When you apply the power, do not touch the loco to restrain it from moving. For a steam loco, hold the tender, and for diesels and other self-powered cars, couple on an unpowered freight or passenger car and and use that as the "brake handle". If the wheels slip when power is applied, your motor should be okay.
If your loco is equipped with traction tires (an abomination, in my opinion), be very careful when trying to add weight. Depending on the tires, slipping could be erratic, causing the loco to bounce as it alternately grips and slips. This is not good for the motor or the tires, as the latter can be stretched out-of-round, causing the loco to run with a pronounced "limp".
More important than the amount of weight added is the necessity to keep that weight balanced, and there's a little more info on that HERE (you need to scroll down beyond the original post).
All of my locos have had weight added, as my layout's many grades and curves offer an almost constant challenge to any locomotive attempting to move a train. When you've reached the point where your loco has no more room for extra weight, and, even with drivers slipping, it won't move the train, it's time to invest in more locos. Wayne

Thanks, Wayne. This is helpful. I'll try to test the two main locos (to which I've now added extra weight) tonight, to see if their wheels slip. All I can say at this point, is that they are performing much better with the added weight. I think I've added roughly 6-8 oz. per loco.

Rob