
You can normally purchase proof Silver Eagles directly from the Mint, except they have been discontinued for this year

There are plenty of perfectly reputable sellers on
ebay, just pay attention to feedback, quality of pictures, and return policy. If you have to be absolutely certain is is authentic, you could purchase one certified by a third party grader, aka "slabbed".
There are currently three types of Silver Eagles. The bullion version is the Eagle for silver investors. It is only available through dealers and tracks the spot price of silver. They have been minted at West Point since 2000 but do not have the mintmark.
The burnished Eagle was introduced in 2006, it is sold by the Mint for collectors. In Mint terminology, these are the Uncirculated Silver Eagles. The dies and planchets receive a special preparation but not to the extent of a proof coin, the best description is a soft satin finish. These Eagles do have the W mintmark, the 2006 is fairly pricey but the 2007 and 2008 are not much more expensive than the bullion version.
The proof version has been minted at West Point since 2001 but there was also a special one-off mintage in 1995 of 30,125 Silver Eagles to be included in a 10th Anniversary Eagle set. That coin is the key date of the series and is worth close to $3000. Fortunately, almost all of the other W proofs can be had for less than $50.