1) You can setup an automated feed using my application
2) You can create a Google Sheet, simply go to Google Merchant Center > products > feeds > click on the plus sign > choose Google Sheets > then choose template and build up your feed
3) Use Webcrawler, but you need to have structured data on your site
Each of these has there benefits and negatives. I recommend researching all 3 before committing as you may discover roadblocks on certain methods.