Large photo frames if one is referring to the width of the molding are not something that can be purchased cheaply and off-the-shelf. Yes, many molding shapes are manufactured but without knowing of which supplier sells which frame and without access to the catalogs its a tricky business to select the appropriate molding for your photo.
Large photo frame suppliers
Your local picture framer, with in-depth knowledge of suppliers should be able to order you a molding at a very reasonable price. Don’t forget your local framer wants your business and will be only too willing to offer advice even if you only want to order a frame and not have it assembled too.
Buying online frames
Online there are many frames for sale but it is a risky business buying something without first having set it next to your photo to see if it works aesthetically and stylistically or not.
The best advice I can give is to ask you local framer for help in choosing a molding and let the framer order it for you.