The idea is to buy the square section, not make it. Aluminium extrusions are mass produced and fairly inexpensive.
To save some time, I went to the glass shop across the road and got a quote. They did not have 30mm*30mm, but both 25mm*25mm (1 inch square) and 38mm*38mm (1.5 inch square) are available. Both sizes are available in 6m lengths with 2mm wall thickness.
Prices are as follows:
25mm*25mm * 6m : ZAR250.00 (USD $30.00)
38mm*38mm * 6m : ZAR390.00 (USD $46.00)
After being cut into 10cm lengths, that works out to less than $1.00 per heatsink. Solid bar would cost much more, because of all the extra metal.
Bear in mind this is the price I would pay here in a small rural South African town. I'm sure you could get a lower price where you are.