Yes, you're hoping to find those nails. And they're there. And they're not as hard to find as you'd think, at least they weren't in my house. You just wave the magnet around a ~16" line back and forth moving up or down vertically until it sticks. Then you can find the next stud by moving 12" or 16" to either side. Seriously just takes a minute.
Here's the top selling stud finder on Amazon I mentioned, it's a magnet in a plastic case, nothing more. 325 reviews, 4.5 star average!
It's usually cheaper but Amazon is sold out (so the price shown is a "marketplace seller" price).
Never underestimate the strength of a good rare earth magnet. If you had two 1/2" blocks of it and stuck them together you would not have the strength to separate them. They'll distort the picture of a tube TV from a foot away.
You can pick one up on Amazon for $8. Or just walk into the Ace Hardware or True Value.
I had this concern too, but if you have never used a magnetic wall stud finder, you might be surprised. link
This has a neodymium magnet in it, and by itself it's pretty heavy. The way it works is you find a stud by finding drywall nail in the wall. Anywhere they nailed the drywall in, they nailed it to a stud. Thus, you can find a stud by finding a nail. To find a nail, you use a magnet. If the wall stud finder sticks to the wall, you found a stud. And let me tell you, they absolutely do work. So I thought if you can hang this magnetic stud finder off of the head of a tiny nail through the paint and drywall, then I figured these ties would hold the cables no problem. Turns out I was correct. These are also neodymium magnets, and it takes a good 10 lbs of force to get them to separate from a flat metal surface. That's more than enough to hold up quite a few cables, especially when you consider you're going to be using at least 4-5 to route a single bundle of cables. That means you'd need to apply 50 lbs of force across the entire length of the cable, and as long as you aren't tightening down your zip ties (leave them loose for f-s sake people!), then you're not going to apply that much force. That is, unless you're wiring up a data center. But if that's the case you're on the wrong sub =)