$15?  Damn that's a nice find--where' at?  Let people know.  I already beaten it back at launch and yeah I love Mario RPG series but the best has to be the DS M&L:  Bowser's Story
I got it at Gamestop when I was picking up my Bongo, drove about 111 miles to get a 2 dollar Bongo but I also had a Birthday to to go.  Yeah I actually had a good month this month games wise.
Picked up:
GameStop
Donkey Konga 1 and 2 plus Jungle beat for about $15
Got Geist for $4
A New copy of Elite Beat Agents for $4 (Which I just bought to randomly give someone because I like the game a lot.  Ironically I got my chance because my friends 6 year old got a DS for his birthday and I was like this is his now.  He got a major haul from us 3 Toy Story Action toy sets and EBA.)
Paper Mario: Thousand Year Door $15
Toys R' Us
Super Princes Peach new for $35 (Believe me that is actually a saving compared to what it's going for used)
Then half off Spirit Tracks so $17.50
eBay
I also scored Ouendan 1 & 2 but they where $20 and $15 respectively.
Its been a good month games wise but my wife is getting on to me for spending to much... So this will need to tied me over for a month or to methinks.  Though it probably will.  I'm now on the lookout for Super Paper Mario again.  I've beaten it, rented through GameFly, but I don't have a copy.  Wouldn't mind the M&L series either.
On a somewhat related note I'm sort of sad I don't see the audience and stage Mechanic in the 3DS Paper Mario.