Where can i buy this size girder ??

[Revs1972]

e5c0rt-mk1 - I've sent you a pm ( if you are still looking for that UB section)

 

[e5c0rt.mk1]

Struggling to find the correct i beams.

Think about replacing them with 100 x 60 box section 3.6mm thick, how much weaker would the box section be compared to the i beam?
Or would 5mm thick box be a more suitable replacment.