I would read the product's label, measure how large of an area you need to amend and calculate how much you will need to use and how often you need to reapply (you always have to re-apply with some regularity in order to keep the soil from returning back to its original alkaline state).

Four pounds should be fine for most people with few shrubs; will probably last years but again, it depends on how much of an area you need to amend, how much of the product you should use for that area (per the label directions) and how often the label says you should reapply.

I have had some difficulty finding agricultural sulfur here in Wisconsin, as well. I finally had to go to a farmer's cooperative feed 'n seed located in a small town 10 miles east of Madison, where I was able to buy a 50 lb bag, at 25 cents per pound, approximately. The agricultural sulfur that I am using is 90% sulfur, and 10% clay. The clay is supposed to help it dissolve in the soil. In the past I have used prilled sulfur in 4 lb bags, which sold locally for about 3 dollars per pound. We have 20 blueberry shrubs, and I needed too much sulfur to pay that much per pound. You can use agricultural sulfur, prilled sulfur, or horticultural sulfur, successfully. Horticultural sulfur is ground to a powder, but it is still sulfur and has the same effect. Here in Wisconsin, sulfur is used on fields that grow alfalfa. The alfalfa is grown as feed for cows, and I think that alfalfa is a high protein feed. Certain essential proteins contain sulfur, and that is why growing alfalfa depletes the soil of sulfur, eventually. I think it is also used on fields that grow corn and soybeans. For a few azaleas, a 4 lb bag of sulfur should last several years. If you end up with 5 or 10 shrubs, then you might consider looking for a larger bag, at a better price.

Found some Espoma Organic traditions Garden sulfur at my local Lowes . Hope it works.