Via eBay; up to 50% off - Forever Stamps.
@Lyric @Erica McPhee Do not buy forever stamps on eBay (or anywhere else) that are listed at anything close to 50% off--they are counterfeit. The Chinese and others have been flooding eBay with counterfeit stamps with increasing frequency of late. They look just like genuine US stamps but are on slightly different paper and, most tellingly, are not tagged. For 40+ years, the USPS has tagged it's stamps, which is an application of ink that can be read with low frequency UV light but is invisible in visible light, which is how the sorting machines know where the stamp is on the envelope (and not be confused by various stickers, etc.), to orient the envelope cancellation.
I know all this, as I recently bought a few sheets of the African daisy international postage stamp for a recent exchange (I had some from the PO but needed more), and tested them with my low frequency UV light, and revealed they had no tagging (different paper, also). I reported this to eBay, and they said they'd look into it, but probably didn't, as they make money on the thousands of counterfeit items--shoes, artwork, so much more--whether or not it's counterfeit. I also contacted the fraud division of the USPS and they were, naturally, quite interested, as this is a big problem for them. They sent me a package of forms and requested all the info I had on it, including the photos I took of the bogus stamps.
Now some stamps on eBay are legitimately sold at a discount--namely older, lower denominations which most people don't use and that are not rare. Discounts can range for 10-25% sometimes. Under US Stamps, search "postage" and you'll see what I mean.
Here is a photo (click to enlarge) of a real stamp (top) and one of the counterfeit ones (bottom). See the tagging around the center of the top one? And the difference in the paper? Even the shape is different--the real stamp has a wavy edge, the fake one is perfectly round.
Take care.
