First off, Welcome to ECN! Personally, I would have thought something like a supply house would be your first port of call? Just one thing to be aware of, buying things like circuit breakers on-line opens you up to quite a bit of liability, whereas, if you get them from a store, you usually have some sort of "come-back", should the product be found to be faulty or even worse counterfeited.
I suppose it depends on who you buy them from. There are certainly some reputable companies selling name brand products (I have hod good luck with Dale Electric and there is always Amazon) but there is also a lot of value in supporting your local B&M store. If the local supplier goes out of business because you can beat his price online, where will you buy something you need today? I can see the value of buying "stock" items online in bulk but that is also what keeps your local supplier's lights on. It is a delicate balance between keeping your supplier healthy and remaining competitive yourself.
I have to agree with Greg 100% on supporting the local supply houses, be they of the corporate groups, or the slowly vanishing 'mom & pop" shops.
You can't call Amazon 'after hours' to get something you need in an emergency! But, most mom & pops and the corp houses offer 'emergency' services. (Perhaps for a price, but if you are a steady customer, you should be OK)