I haven’t done it myself but you can bolt a large steel plate to the inner beading of the tire and attach a rod to the centre, then simply use plates to increase the weight. Much like a big round sled attached to the tire inner.
At 150 lbs, it’s probably not very wide. But one thing I’ve seen done is to just add regular-sized automobile tires to the inside. But, again, at 150 lbs, you won’t get very heavy with this method.
Find any company with the word ‘tire’ in their name. They have to pay to dispose of the tires. If you call and ask for one, they should be more than willing to give it to you. Just remember, if you decide to return it. it will cost you.
For extra weight for anything, look for piping fab shops and ask for surplus. The same thing applies, but steel is a lot heavier.