Any suggestions for guttering source/ supplier

[Modernista]

I need to buy two lengths of (preferably) square section (preferably) metal guttering to go round the two edges shown in the photo but it includes the need for a T junction at the corner to allow water to run on to gully with an open end. I've seen a few of these 'T' pieces sold as plastic conservatory gutter fittings but can't seem to find anything else as standard. Might need to be custom made if necessary? Any suggestions welcome. Metal preferred in case it gets stood on etc.