Surprised no one has mentioned this, what you actually want is a one-man or two-man crosscut saw. Properly sharpened (which is a big deal) they cut much easier than a bow saw. One-man crosscuts range from 3' to 5', while the two-man saws range from about 4-1/2' to 8'+. Ideally, you want the saw length to be 2x the diameter of the tree +6", so in other words a 1' tree needs a 2'6" saw, a 2' tree needs a 4'6" saw, etc.
There are very few of these being made new today that are worth anything. The very best crosscut saw blades are tapered, so that the cutting edge of the blade is thicker than the back of the blade -- this makes the saw run easier in the cut. The tooling to produce taper-ground saw blades economically all disappeared around 1950 or so, so the best saws were made prior to that. So, you're probably looking at ebay or craigslist -- unless you already have one hanging over your fireplace?
Try craigslist, you can probably find something inexpensive. Expect to pay a little for a good sharpening job, but unless you use it every day, it won't need to be sharpened that often.