If you need new ones, you can get them using GM p/n 461854. Need 4 per car. List price is $11.35 but you're likely to find them cheaper pretty easily. 11516073 is the p/n for the nut to go with it. Need 4 per car. List prices is $2.44 each for those.
You can buy aftermarket at places such as ebay, but I have no idea about fit and quality.
This set of 4 Upper bolts are Made in USA the proper 7/16" serrated type and a Grade 8 bolt. Original style head markings are correct for most cars, and they even have the correct black phosphate plating!
www.ebay.com
They do have the serrations and I have had luck with a brass drift punch, or basically a rod, then mini-sledged them out. They don't simply go quietly into the night. Of course, the engine was out at the time making it so much easier.
Agree, you shouldn't have to remove the bolts to remove control arms. But if you have headers or other weird engine/exhaust combinations, it could make changing sh*t a bit more difficult.