For these daily t-shirt things I usually use this:
http://tshirtroundup.tumblr.com/
Haven't seen anything I wanted to buy in months though.
I don't remember if you like or keep up w/ NISA, at all (or get their "Prinny Bomb" e-mails)...but, when they announced that there's going to be a (9 day) delay in the release of
Hyperdimension Neptunia: Victory - they were also showing some (Neptunia-series-related) pictures of t-shirts for sale...
Personally, though - I'd have to say that t-shirts, other clothes, pillow-cases, etc are the 'collectibles' that I really have the *LEAST* amount of interest in.
*MY* personal favorite, "top-3" 'extras' are (in order):
(1). A nice strategy guide,
(2). A nice (and *complete*) soundtrack, and, I guess that 3rd would be
(3). things like art books w/ interviews, Q&A, comments, etc from the team and/or bonus "making-of" discs w/ extra content on them...
...How about the rest of ya?