I understand that the StackExchange is not a product recommendation service, which sort of invalidates your question for a specific brand.

A quick search of ground lugs returns many results of rectangular shaped lugs identical to the one in the photo you've provided as well as a few variations. If the one in the photo does not have a perfect flat bottom, one might expect that it would flatten under the force applied by the bolt through the hole in the tab.

Alternatively, one might expect that the application for which this product is designed takes the non-flat aspect into consideration and allows that the surface area which will make contact is sufficient for the level of current expected for this product.

If your application is non-compliant with these factors, consider that you may not wish to use this specific product.

If it is necessary to absolute flatness, filing, sanding, grinding or machining may provide the results to match your requirements.