A unique ISRC code is assigned to each song you release publicly. Permanently encoded into your recording ensures that your song is identifiable so that collecting societies can claim royalties on your behalf. Most digital distributors provide these for free.
However, you can be your own ISRC manager and manage your own ISRC codes. Here are some of the agencies around the world from whom you can obtain your own registrant codes. See the full list of agencies.
In the U.K. they are free to PPL members; in the USA there’s a one-time fee of $95 from the RIAA viausisrc.org. If you have some suggestions or experience buying ISRC codes, please share with us all.