Im doing my DS's room at the moment in a sports theme. Ikea is awesome! So much cool stuff and ideas
Bunnings also have some cool light shades and night lights. Reject shops sometimes have some funky knick knacks for decorating ideas.
If your crafty check out pinterest for ideas, or google image 'boy room ideas' and make your own cheaper knock offs
I bought a heap of little but different sized and type balls, joined them with fishing line and he has a ball wall border that I think looks pretty cool.
Markets are wicked but so good too The wooden letters for names, bunting, so many gorgeous but expensive things.
I have to admit DD's room was easier I had to stop myself but still having fun with DS's