While we don't currently offer a matte photo label material, we do have a couple different products that may be suitable for your application. They differ pretty significantly in terms of appearance and performance, so you're best bet may be to try sample sheets of both to see which one you're most comfortable using for your project.
Our
standard white matte label material features an uncoated, paper-based finish. The uncoated finish on this material means that it may not reproduce color photographs with the same type of image quality you've experienced when using specialized matte photo paper. We do have a large number of customers using this material for product labels though, and it is definitely possible to achieve attractive full-color print results.
Our
weatherproof matte inkjet label material features a film-based material with an inkjet topcoat. I know you mentioned that outdoor exposure would not be a factor, so a weatherproof product may seem like overkill, but this material option should provide better color photo reproduction if you decide you're not happy with the print results on the standard white matte labels.
I'd definitely recommend requesting sample sheets of both of the products linked to above and testing them with some of your artwork to determine which product works best for you. You can enter a free sample request by visiting the link below:
https://www.onlinelabels.com/general/SamplesSelectItems.aspxI hope this helps. If you need any additional details please just let me know.