Aspirin is called a blood thinner but more appropriately it is an anti-coagulant.
Simply put, it blocks the clotting cascade. Does this equal more blood flowing to muscle in healthy folks, unlikely. Even so, there is way more oxygen in the blood stream than the muscle can extract. Extraction is improved by training.
It is more likely that aspirin effects arm pump by its anti-inflammatory properties, possibly interfering with the inflammatory response associated with fatigue, blockade of pain for muscle pain fibers, and reduce chemical mediators associated with inflammation that could impact the muscle's contractile ability. If it works for you than have at it, I just wouldn't over do it as the potential side effects exist. Ibuprofen may be a little safer and easier on the gut, although the mechanisms of action are different and may not work as well. I for one, wouldn't want to take a blow to the head loaded up on aspirin. :o
As far as propel goes, it is available everywhere! Just take some water and break a multivitamin into it and presto, you have propel.
Good Luck, Keith