Complete 404 Re-directs Post-website Move

We recently moved our ecommerce store from one platform to another. While many of the re-directs from the old site where handled correctly we are still have 25,000+ crawl errors showing in webmaster tools. Some of these errors are inaccurate, however we would like a freelancer to go through and correct the URLS that are directing to a Not Found. A little research on the new site is required to do this the most accurately.