Non-consumer focused steel suppliers will often sell in 6m increments. Others (such as metalsupermarkets) will sell any increments with no minimum order. Some suppliers will also cut your steel for free or for a small fee ($1/cut).
Check your local listings to see if you have a steel supplier near you. Here is a list of suppliers used by builders so far:
https://www.scottmetals.com.au/rhsptd
https://directsteelsales.com.au/ Melbourne
https://steelsolutions.com.au/ Melbourne
https://www.fhbrundle.co.uk/products/300575503__R_H_Section_75_x_50_x_3mm_x_7_5_7_6m_Grade_S235
https://www.fixmetall-shop.com/
https://www.ijzershop-acac.be/ijzer/637-vierkante-buis-80x80x5.html
Daniels metalen helmond (cheap but don't cut to size 6M length)
Another option is to try finding a local steel supplier or manufacturer that uses a lot of steel. This one may have some ‘waste stock’ :
https://www.metalpoint.nl/p/Restverkoop