Noise-isolating headphones are great, but if you have a favorite pair of earbuds that you just can't part with, you can turn them into noise-isolating earbuds of sorts that block out outside noise without spending a bunch of money.
Most cheap earbuds that advertise noise-isolating capabilities don't do a very good job at actually blocking out outside noise. And even if they did, they still might have terrible audio quality. You can go with a known brand that specializes in this area, like ISOtunes or Plugfones, but the cheapest pair of wireless earbuds from ISOtunes cost $60.
Luckily, there are several ways to add noise isolation to any pair of earbuds, including your favorite pair, that's cheaper than the specialty earbuds from the above companies.
Buy Specialty Foam Tips
The best way to turn your normal earbuds into true noise-isolation earbuds is to buy some specialty foam tips, and Comply is perhaps the most popular company that makes them.
Its foam tips are made out of a "viscoelastic memory foam" that's very similar to what ISOtunes uses in their OSHA-compliant earbuds. They're similar to earplugs in that you compress the tips with your fingers, slide the earbuds into your ears, and wait for the tips to expand to completely seal your ear canal. I wear these puppies during plane rides and even while I mow the lawn, and they work really well.
Comply has several different models of foam tips, but their Noise Isolation line has several different versions that work with pretty much any pair of earbuds. You can simply click on "Find Your Fit" to see which ones to get for your specific earbuds. A set of three will cost you $20, but those should last you a good while.
Retrofit Regular Earplugs
If you want a dirt cheap alternative to specialty foam tips, you can retrofit some regular earplugs that do mostly the same thing. And it's actually what I did years ago before I even knew specialty foam tips existed.
It does take a bit of work, though---you have to cut down the earplugs to make them shorter and punch a hole in the middle so that they'll fit onto your earbuds (and so that sound can travel through). This can be done using a leather punch tool or something similar, but I actually just used a drill bit and carefully drilled out a hole through the center of the earplugs.
It's not the best method, as the earplugs don't latch onto your earbuds very well and tend to stay in your ear whenever you remove your earbuds. So that's kind of a pain in the rear end, and it's highly worth it to just buy some special foam tips.
If you're on a tight budget, though, this alternative does at least work pretty well. Plus, it's really cheap---like $9 for 50 pairs cheap.
Make Your Own Custom Ear Molds
This isn't for everyone, but if you really want to take things to a new level, creating your own custom ear molds that shape the inside of your ear perfectly and completely form a seal is the ultimate way to go.
You can get a kit that includes everything you need for about $10. It takes a bit of patience, though, since you have to mix putties together, push the mold and earbuds into your ear, and then wait for about ten minutes for the mold to set. Then you have to let it cure for a couple of hours.
We have a guide on the process, so if it's something you're willing to tackle for the sake of having some sweet noise isolation, this can be a great way to go.