I order from Gardeners Supply and sometimes other companies. Where I buy depends more on who has the specific thing I want rather than a feeling that the quality will necessarily be better. Sometimes you get good leads on products from other gardeners and these products may be carried by the online vendors and hard to find locally, Not everything I have bought from online garden suppliers has turned out to be high quality. For your specific example, you might be better off comparing large hooks sold in the hardware department

Mine are wrought iron from Agway. The local high-end nursery also carries them.

I've never ordered from Gardener's Supply, but I have visited them several times. In general, I was underwhelmed by the quality of items in their store. I used to order garden things from Kinsman, but not recently.

I had a bad experience with Gardeners Supply a couple years ago, and after that I put them on my "Do Not Buy From" list. If I remember correctly they sold me a pair of boots that didn't fit. The "foot part" of the boot was the correct size but the "leg part" was sized to fit Twiggy. It wasn't worth the hassle or expense of sending them back, so I cut off the uppers and wore them as shorty boots.

I second the advice to visit a hardware store or farm store to buy sturdy cast or welded steel or solid brass hooks and other hardware. As to online sources, I've had excellent luck buying hardware from this place:

http://www.cdwplus.com/singlemenuM.html

I haven't bought any larger garden hardgoods online, though occasionally I will buy tools. Even in my relatively rural area I can usually find something solid I can use to fit the purpose at a garden center or nursery, farm store, big box store or through used goods venues such as yard sales, charity shops or Craigslist. For something like plant hangers, I would be likely to check the farm and hardware stores first. If you don't find what you want, you can also see if there's a local blacksmith who can make what you want. I am generally fairly frugal, but for me part of that can be spending a bit more on something that will last my lifetime and beyond, whether it's a woody plant or a good quality piece of wrought iron.

A tool I love is a Cape Cod weeder. I lost my first one and the second one is a different kind but similar. I bought an inexpensive version when I thought I had lost #2. It seems good enough. I don't understand why local garden centers don't carry some good hand tools. I don't see anything like a Cape Cod or cobra headed weeder although the reasonably priced substitute was found at Stone Falls in Henniker NH. I would have ordered a replacement. I would go to a building supply or good hardware store for things like hooks. In our area we like the Barn Store in Salisbury NH and the Farm Store in Henniker. Agway is pretty good.