Well from one old house homeowner to another I can give you some info.
Lowes and Depot carry some standard types of radiator valves. Youwon't find all the options you may need, but sometimes you luck out.
The local plumbing house will almost always have what you need and good advice from the oldtimers at the counter. The prices usually are fair and you can get the product immediatley, or worst case in a couple days depending when their deliveries come from the warehouse.
The last place the internet gets you whatever you need. My favorite site is pex supply .com