Here's my list of suppliers (See more examples of the products at my store)
Stickers: Sticker Mule (free shipping within the U.S!) I really like the matte finish and thickness of their vinyl stickers. I also got magnets from them which aren't terrible, I found them a little thin, but they're holding up fine.
(magnet left, sticker right)

Charms: Zap Creatives I got their orange fluorescent ones when they had an introductory deal. They're pretty cool. Charms are nice and thick. I also got stickers from Zap once, they're glossy finish but are much thinner than Sticker Mule's.
Enamel Pins: Like bear, I had my first pins made with GS-JJ. There weren't any problems with my order and the pins all arrived in great condition and quality!
I also liked that they had options for shipping for those on a budget, for a longer production and shipping time you can save up to 40% off on your order.
I am currently waiting on pins from The Enamel Pin Factory to compare. They are UK based so I have to factor shipping into my order. I'll update this post when they do arrive.
UPDATE
My pins from The Enamel Pin Factory came and they are wonderful!!
What I like about tEPF is that if you order a set number, they give you several extra pins on the house. I ordered 100 and they sent 110. They also manage your order through Trello in which they invite you to join to send them your artwork; it's nice seeing all the specs in one place. I must say, the quality IS better than GS-JJ (I had fewer pins that I would sell as seconds/B and C grade), but given the price difference I would hope so. For example, a silver-plated 1in 100ct standard color pin order would cost you $122 at GS-JJ and $207 at tEPF (with shipping included, and at the current pound to dollar rate).
So I suppose it comes down to what you need in your pin. GS-JJ has metal colors that tEPF doesn't offer, like rose gold and copper. tEPF does up to 5 colors at no extra cost, whereas GS does 4. tEPF can do a back stamp. I encourage folks to check out each site for themselves and compare.
Printables
Comics: I used Greko Printing - Comix Well Spring and I think they're comparable to Kablam (at least the Kablam produced issues I have from other comic creators), but CWS's cover are slightly thicker which is nice.
For my longer book (like 90 pages), I used CreateSpace and I think they do just fine with black and white printing especially for the price. I can buy my own copies for under $3 each and you can mark up the price to sell through Amazon.
Business Cards: Moo is my go to! They have extremely nice cards and I've also gotten postcards from them before in equal quality. Plus their packaging is amazing!
If you're on a budget, Vistaprint works just fine. I use them to make my backing cards for my pins!
Other printables: Cat Print I made my wedding announcement/invitations and thank you cards through them. I like their sparkly paper lol.
