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:
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’ :