The most common device to draw such a line on a golf ball is a product called the "Line-M-up." It's essentially a piece of plastic that you snap on a golf ball. It has a slot where you can draw a straight line on the golf ball with a permanent marker such as a Sharpie pen.
You can buy the Line-M-up at most golf stores. At Golfsmith, it costs $8.99. Yes, $8.99!
While that may not sound like much to some, it's quite a lot for something that looks like it cost about 5 cents to manufacture. Sure, it comes with a Sharpie, but what does that cost, another 20 cents? Assuming a standard 100% retail markup, I estimate that the Line-M-up wholesales for about $4.50. To be generous, I'll assume that the Line-M-up costs a total of $.50 to manufacture and package. That means for each Line-M-up sold, the manufacturer makes $4.00 in profit, or a 800% profit margin!
I'm not against anyone making a buck, but to me this is grossly excessive. As a businessperson, I refuse to buy this thing based on business principle alone. But that doesn't mean I don't draw a line on my ball. For the price of a bottle of Gatorade, I have a ball line-drawing device.
When I wrote, "a bottle of Gatorade" I literally meant a bottle of Gatorade. I just use the plastic safety ring that detaches from the cap of a 20 oz. bottle of Gatorade as a straight-edge for golf balls. Amazingly, it fits on a golf ball almost perfectly! Not only do you have something to line your golf balls, but you also have a tasty beverage to wash down that crusty old hot dog that you bought at the turn. To ensure a straight and balanced line, use the Technasonic Check-Go to pre-draw the line. Just take a look at these pics to see how easy it is to "Line-M-up" ghetto style: