Everyone seems to be having problems downloading the Public Test Realm (PTR) client still. This can be a real problem for some, especially if you don't have access to a high speed internet connection all the time. You can read the PTR patch notes multiple times over and still not be finished with the download.
WoWWiki has a great list of a few mirrors, and I've linked to the most trusted ones I use all the time after the break.
Please keep in mind that it is always possible, although highly unlikely, that one of these files might contain a virus. You should be running something to scan for viruses on all incoming files, like the free AVG.
See below after the break for the mirrors list.