Why do you want a 16ft, or wider door? Is this a two car garage just for auto's? If so, then allot of homes, millions, were built with two 2x12 and half inch plywood sandwiched between them. It's not code in some areas anymore, but those homes have lasted decades with those headers in them.
Over 16 feet, I would only use a glue lam beam. Price isn't that bad in the overall scheme of things. I think I paid around $200 for the last one I bought. It was 24 feet long and if I remember correctly, 14 inches tall. They come in standard lengths and you cut them down to size.
Lowes, Home Depot and just about any building supply store sells them. They don't stock them, but will sell them to you from the local company that builds them. Here, there is one company that does this, and all the building stores sell the exact same beams. Markup is different for each store, but not a huge difference from what I've seen ordering from three different stores.
They are very heavy, so expect to use a lift to get it into place. I've done them by myself with a hand operated lift. Rental of the lift is just $40 for hald a day and I've never needed one for more then an hour. The lifts look kinda like a hand truck with forks on the front. You crank the handle and up goes the beam. Real simple and easy with a little planning and prep work.
If you plan on putting your tractor in there, you really don't want a standard garaged type door. You didn't mention this, so it might not matter, but I've found that a 10x10 roll up door is just about perfect. Bigger is always nicer, but not really needed. My previous shop had a door 12ft wide and ten feet tall. That ten feet height was awesome. Twelve feet took away from space for the man door and made things kinda tight. On my current shop, I put went smaller and don't regret it at all. I was able to put my electrical panel between my man door and rollup door, which make it real nice to get to. Nothing blocks it!!!!
I also found that 12 foot walls make a HUGE difference compared to 8ft walls in most garages. That extra height makes working on things, building stuff and just plain old storage allot better.
Let us know what your usage is for the garage, and how big it will be, and we can offer suggestions that you might not have considered.
Eddie