I have 2 8gb Micro sd cards for my ttds. Ordered them from mymemory.com.
I can fit around 200 games on 1 card.
The smallest games are like brain training 4mb, the biggest, pro evo 2008 that are 128mb.
The only way you could have 1000 games is to either have 1000 snes games or homebrew etc. No way can these be propper DS games.
No one would be selling carts with games as its illegal and very risky to sell to public. Never belive what people from car boots tell you, if it sounds to good to be true, it is!