What is a "good" strawbale?
The most important characteristic of your strawbales is that they are dry! I do not use bales that are, or have been, wet for any construction project (including mixing in to cob or plasters). To check for moisture content, I start by visually inspecting the strawbales. First, I look at the conditions under which the bales are stored. I make sure they are not sitting directly on the ground (if they are, I let the farmer know that I do not want those bales). I make sure there is air-flow around the bales and that the roof isn't leaky. And I make sure the bales aren't leaning up against metal anywhere (metal will condense moisture and make the adjacent bales wet). Then I start inspecting the bales themselves. I look on the surface of the straw for any signs of mold. It usually is black in color. I smell the bale to make sure it smells like straw (not mold or any other contaminant). I pop open at least one bale to inspect the inside for mold too. I lift a few bales to check if they are unusually heavy. 2-string bales typically weigh about 45 to 55 pounds. If they are very heavy to pick up, it can mean they are super tight, but often means you are picking up lots of moisture absorbed by the straw. Finally I look to make sure the straw is not contaminated with unusual amounts of dirt or detritus. (This is usually not a deal-breaker, but clean bales are much more pleasant to use.)
Next step is to test the bales with a moisture meter. The farmer you are purchasing bales from may have a moisture meter (usually called a hay moisture meter, which is fine) that they can test your bales with. They are several hundred dollars to purchase, so I would try to borrow one unless you intend to use it for multiple projects. Insert the moisture meter probe into roughly the center of a bale and check the moisture content. I do this in several points of each bale and test at least 20 bales. They should read below 18% moisture content as an absolute maximum. Lower is better. (Note that the bales will continue to dry out unless moisture is introduced.)
In addition to being dry, the bales are much easier to work with at every stage of construction if they are nice & tight. To check for tightness, simply lift a few of the bales up by one string. It should be difficult to get your hand underneath the string, and the deflection of the string should be small enough that you don't feel like the bale could fall apart when lifted by the single string. (If the bale is loose, it feels like the string is just going to slip off the side.) The only other visual inspection is to check that the bales are reasonably rectangular. Sometime one string is tight and the other is loose, and you have bales that all curve to one side, which is kind of a pain in the butt when building.
How can you locate strawbales near you?
To find bales I recommend a two-prong approach:
First, check www.hayexchange.com. Click on your state and also check any neighboring states. (You can even expand to further states if you are coming up short.) For each of the states you will see a list of suppliers and what they sell. Many of them may list hay, alfalfa, etc., as the website name implies. But there are also straw listings. You just need to make sure they are not listed as round bales or as mega bales (unless that's what you are looking for). Any grain variety is fine. I think barley is the nicest to work with, as it is bright & pliable (doesn't seem to break as much), but it really doesn't matter.
Second, I recommend looking for a very local farmer, which can mean lower priced bales and often free delivery if they are very close. You can first see if there is a farmer's newsletter for your region. These might be found at an agricultural supply store or someplace similar. Here on the East Coast, lots of farmers simply put up a sign on the road that offers "straw for sale". If you are building somewhere similar, I recommend driving around and looking for those signs.