your file upload limits are controlled by your php configuration (php.ini). if you’ve tried the various fixes found when searching “php increase maximum upload filesize” or similar, then that’s a question for your host.
as far as hosting files elsewhere and a commerce plugin that supports that feature, I suggest having a look at Woocommerce and their extension for hosting files on Amazon S3. It’s not free, but it’s a well-supported and reliable system.