I personally use JP's corner, great customer service and fast shipping. Let me know who you use and why.
mostly my LCS, I get a pretty good deal. I did split a 2 case order (500 holders) with the LCS 3-4 years ago from JP. great deal on a mix of 4 sizes of airtites. also buy odd stuff from ebay
I think JPs Corner and Wizard Coin Supply are both pretty good. No issues with either. For smaller purchases (a couple tubes, a few 2x2 binder pages), your LCS is probably the best bet.
Wizard is good with the ANA discount. Valley seems to be the only place with Intercept stuff right now. I just ordered some IS mint set boxes from them. Good prices compared to ebay.
Online I use JP corner. But whenever I buy coins from my lcs the owner gives me a few freebie supplies. Plus its much easier to get supplies when you can see what you're getting. I have gotten two orders that wasn't quite right already.
There is a local B&M shop near me that has Dansco, and Whitman folders, 2x2 holders, 2x2 flips, airtites, and gloves. I also order from here now and then: http://www.wizardcoinsupply.com/
I'll put your coin in a holder with a label for half of what they charge, and i'll even give you a better grade
I only buy flips, albums and reference material. Albums and reference material I usually buy from eBay or other forum members. Flips, I buy from PCGS.
I have only one local B&M that carries a broad range of supplies and they make sure they profit from it so usually I buy from Wizard mostly due to selection and price and sometimes depending what wiz is sold out and their price I go with coin supply express. They also have Intercept Shield albums. Only bad thing is they ship FedEx and for about 2 years we were having our fedex packages dropped a block or two away. So if you have poor fedex drivers who don't care look elsewhere.
I get flips and such at my favorite B&M shop, if he has what I need. Otherwise I go online. I usually get books from Amazon.com.
I ordered twice from Wizard, both times they screwed up the order. Girl I talked to seemed pretty rude. Used the excuse that's "there's only two of us here". Not impressed. I used Brent-Kruger a few times, not bad, but no longer in business. I now buy exclusively from JP's Corner. I can't speak for their customer service, as I have not had to deal with them (perfect every time). Cheaper than the local guys, get the stuff out pretty fast.