Need some expert advice, so I came here!

I need some information and I hope reddit's gaming experts can help. My son is 11 and has Asperger's Syndrome. His current focuses (obsessions) are Minecraft, trains, and legos. I have two questions: A) Is there somewhere I can get him a Minecraft premium account at a discount? (Long story short, job losses and other factors make the budget very, very small and I just can't swing it right now). I've searched far and wide unsuccessfully and I hoped perhaps someone here might know of something I could try.

B) His grandmother bought him and his older sister a DSi XL for Christmas and I'd like to know if there are any good games related to legos or trains. Or Minecraft, for that matter. (His sister is 14 and loves Pokemon-she would love Pokemon Pearl and Black 2-I've seen the prices and that ain't happenin'! lol) Are there any games that are inexpensive? Is there a place I can buy used games?

Any help here is very much appreciated. My son has done very, very well this year in dealing with school, bullies, math, starting a new school, starting puberty that necessitated a medication change (he'd die if he knew I were spilling this) - anyway, I wanted to do something special for him for Christmas. My gaming experience lies with NES and the arcade version of Galaga (which I can still kick ass on, thank you very much!) so I'm not too current on what's available these days.

Sorry for the wall of text! Thanks to everyone!