I cannot speak globally, but any optometrist I could go to is a Doctor of Optometry, and it is he who writes the prescription. He may (and typically does) have his own retail shop at which the prescription can be filled, but I can take the prescription to any other retail shop. The typical "vision center" (e.g., Sears, LensCrafters, Wal-Mart, Costco) is a corporate-operated retail facility with an independent Doctor of Optometry on the premises under contract.
That being said, I agree with Silverquill that such would be an "every other corner" type of entity bordering on ubiquitous.