An alloy often used for flashlights is also sold here: http://www.aluminiumwarehouse.com.au/index.php?route=product/product&path=1001_126&product_id=421
6061 refers to the chemical composition, and T6 refers to the heat treatment.
The aluminium bar from the link is probably low alloyed aluminium, as nothing is mentioned about chemical composition. So strength is very low. But if you want to make a heat sink that should not be a problem. Low alloyed aluminium has generally slightly better thermal conductivity than higher grade alloys.