As of May 16 2016 a GTIN (Global Trade Identification Number) which can be a EAN, UPC, IBAN, JPN is going to be required for the following countries Australia, Brazil, Czech Republic, France, Germany, Italy, Japan, Netherlands, Spain, Switzerland, United Kingdom, United States.
The general rule is that if a manufacturer has assigned a GTIN ie a bar code on the product than it is now required to add these values to your data feed.
Google is always trying to improve it’s services and the GTIN requirement is enforced to ensure the ad quality is increased. Which means that if you completely fulfill all data feed requirements and policies that your ad quality will increase and thus have better ROI.
If the manufacturer has not assigned a bar code than you are not required to add the GTIN value. You can simply assign Identifier Exists to False in the Google Data Feed Attributes.
If you are lucky you might be able to get the GTIN’s from the manufacturer, try to pursued them that it is in there best interest to supply you the data as it will increase sales for both yourself and them.
Barcode Scanners :
You can digitize your bar codes by scanning them using your smartphone. There are a ton of apps out there that can do this very easily
Barcode Databases :
There are also bar code databases online that supply all known bar codes, by using the product id or title keywords. You can search on Google for barcode lookups
Your individual products will be disapproved, if a large amount of products have similar issues than a possible account suspension can take place.