I like Potomac Distribution for booster boxes. Each box is $90 during Pre-Order and after the set comes out, and they ship 2-3 days prior to release date so that you get your boxes on the release date. They offer free shipping on orders of $150+, which I normally get since I order 2 boxes everytime. They also offer coupons every month, so you can get free stuff with your order. I got some free Premium Toploaders with my Flashfire boxes. I also like that their boxes cost $90 for about 6 months after release date, then only go up in price by like 1 or 2 dollars once the next two sets after that particular set come out. For popular sets like Flashfire, that's a good thing, since I've already seen stores and eBay sellers trying to raise the price to around $100 per booster box.
For singles, I buy only from eBay, and for two reasons that IMO are really important. 1)Card Condition. It's not a secret that eBay tends to side with the buyer almost 100% of the time when there's a dispute, so if you get some cards that are considerably below the described condition, you can always open a Significantly Not as Described Case, and PayPal will almost always refund your money. 2)eBay bucks/coupons. Not sure how many of you are aware of this, but everytime you buy something on eBay (assuming it qualifies for this bonus), eBay gives you back a percentage of the total price you paid. They call this eBay bucks. Basically, if you end up accumulating at least $5 in eBay bucks during a given quarter of the year, they'll e-mail/message you a certificate that you can redeem on anything you want on eBay, once the 1st month of the next quarter comes around. If you buy frequently, you can get a pretty good amount of money back, and I think the max is $200 that they give you. They'll also randomly send you coupons for $x off your next purchase, which can stack up with your eBay bucks certificate for a good amount of saving. Just three days ago, I received my 4th Discount Coupon from them this year. The first was for $5, but my last three have been for $20 off. I currently also have around $30 in eBay bucks, which will be sent to me once this month ends (and I still have 12 days to make that amount grow
). Unfortunately, I only have until 6/30 to claim my Coupon, so I won't be able to use both Coupon and eBay bucks simultaneously this time :/
Another good thing about eBay is that you can get a steal every now and then on good cards, but it's a matter of being lucky enough that other bidders/buyers don't see the item you're interested in. Just last week I managed to win a Mint Shinining Charizard for only 40 shipped, and only two other people bid on it, with the starting price being $35.Not bad.