Olympic weight plates for barbells. They're widely used, so competition has brought the cost down, and they're easily available in useful increments. I currentl

Olympic weight plates for barbells. They're widely used, so competition has brought the cost down, and they're easily available in useful increments. I currently see 4x 10lbs for <$50 on Amazon. That works out to 2,53 Euro per kg. So cheaper than euro cents. They may not have the exact shape you need. The scrap steel probably didn't cost cents per kg when it was sold for its original purpose. You are paying for a useful shape. A professional equivalent of weighted vests are ballistic plate carriers. Real ballistic plates can be fragile and expensive, so options for exercising in (or milsim games in airsoft etc.) include expired (and failed to re-certify) real ballistic plates, made for purpose training plates... or plate shaped sandbags!