We're in the middle of making this happen.
The first step is getting your labeling ready. To do this you'll need to get a UPC Code assigned so the stores are able to scan the package and have a Nutritional Analysis done so that your product will meet FDA requirements.
Once that is done you can have labels printed up. Then you can self-bottle with an approved canning setup if you have a commercial kitchen or rented commissary available. This is often a good idea for getting started, because the copacker is going to require a large initial order and you may not want to commit to 100 gallons of sauce that you don't know will sell.
The final step of hiring a copacker can be pretty tough, especially if you are picky about your sauce recipe. Copackers will take your recipe and try to recreate it with the bulk ingredients they regularly stock, which may or may not accurately reproduce your product.
For more information, check with your local extension agent or health department.