Stop, think, and if you have something to add simply edit it into your post. Rapid-firing one-sentence posts asking wildly disparate things is not the way to go.
The best way to get bricks on the cheap is to check yard sales, flea markets, eBay, and Craigslist for bulk LEGO lots. Be wary, as you may often find the collection full of clone bricks: cheaper LEGO look-a-likes such as MegaBlox or Best Lock. From there, it's a matter of putting pieces together to build the ships you want. Research SNOT (studs not on top) to make your creations look very good with very little effort.
If you want Halo-specific pieces, MegaBlox has a line of fairly cheap unit and weaponry packs, as well as vehicles from the games. Unfortunately, they are MegaBlox, so the vehicles rarely stay together without tape while under constant play, and the minifig have a habit of breaking at the joints. Still, there's that option.
You can buy specific bricks off of LEGO's Pick-A-Brick system, but you can potentially spend a lot of money for little gain (except for the fact that you get precisely what you want if it's available).
EDIT: I totally spaced out Brick Link. Prices and customer service experiences vary, but it's more-or-less the eBay or Amazon of LEGO pieces. Need three black dragons from the Dragon Knights set or twenty clear 1x1 bricks? Check'em out.
It's a secret to everybody.