Gareth,
I assume that you are in the UK, but if you get really stuck, these people do I-section and H-section in brass.
Their web site, from where you can download their (very extensive) catalogue:
**LINK**
The stuff you want is on page 53. I've not used them myself, but found them while living in France.
John
edited to remove bad link and add this one:
http://www.octantenligne.com/04537d9dc40f7690c/index.html
click on "PROFILES FRAISES LAITON FORME H, I, Z"
Edited By John Hinkley on 23/03/2018 08:06:10
Edited By John Hinkley on 23/03/2018 08:07:52
Edited By John Hinkley on 23/03/2018 08:09:06