The Wall Street Journal: Captured Russian weapons make up the largest supply of heavy weaponry for Ukraine. 

According to open-source intelligence analysts interviewed by the Wall Street Journal, Ukraine’s rapid breakthrough in Kharkiv Oblast last month helped putting hundreds of pieces of Russian armor into Ukraine's hands. This, combined with weapons taken during Russia’s retreat from Kyiv and other parts of northern Ukraine in April, has turned Russia into by far the largest supplier of heavy weapons to Ukraine. In sheer numbers, it put Moscow well ahead of the U.S. or other allies. However, Western-provided weapons are usually "more advanced and precise," according to the WSJ.