I'm heading to fan expo as Ness this year. It will be a simple costume to put together, but here are my ideas:

Shirt:
A yellow shirt painted over with blue stripes. (Will probably use fabric medium and some acrylic paint for this.)

Hat:
Red cap painted over on the brim with blue.

Baseball Bat:
Party style inflatable baseball bat.

Pants:
Jeans

Hair:
Cropped, boy style

Breasts: (Yes, I'm a girl...)
Bound up in a sports bra and some duct tape. (Ow.)

Got any ideas for how to improve my ideas? (It will be a rough costume, but I didn't spend $10 on it.)

Yellow backpack. Get a yellow backpack. Anyways, good luck with your cosplay! Sounds like it'll look great ^_^

If you have an old wooden louisville slugger bat around it looks just like the bat Ness uses. I've had one around and I've always thought of EarthBound when I see it.

I'm not sure if fabric paints work the same way as acrylics, but you may want to mix a very tiny amount of white in with the blue to make it more opaque (or else you may end up with green stripes). I've also seen someone use blue masking tape on a yellow shirt to make the stripes. That may be easier!

Big stores like Walmart usually have cheapo red shoes in stock. I found a pair of red Ness-like shoes there for $5. They aren't very sturdy, but you could reuse them after the event is over.

I'd say shirtness and hatness.

What's this?
Women?
On my internet?

I'll second the recommendations of Shirtness + Hatness along with a Louisville Slugger.
I'm unaware of the prices of any of them, though, and depending on how cheap you want to go, it may not be the best choice.
All around, though, it seems like you have a solid plan put in place already.

While they're both perfect choices for Ness cosplay, the Shirtness has been out of stock for months and reid can't get in touch with the same guys who custom made them. Also the hatness is $16 iirc and astrangeoneEB wants to make this super cheap.

Check your local thrift stores, you may be able to find a super cheap yellow backpack there along with a possible Ness shirt. I found a blue and yellow striped polo for $3, of course Ness wears a tee shirt but it's pretty awesome anyways.

Guess I am lucky to get them before they where sold out.

Yeah I'm irritated at myself for not purchasing two when they were in stock.

I need a medium and I've been asking people on starmen. D:

Yeah, the shirtness/hatness are expensive.

I found my old pair of winter boots for Ness's shoes. Red with a little black trim. I also plan to cut my hair ultra short to do that as Ness.

Hopefully, I can find an old yellow knapsack for the rest of the cosplay.

Also, I do not want to carry a real baseball bat around. Convention rules and the fact that it's going to be a heavy object.