If you have a Windows computer, you can create “acidized,” stretchable files for free

There are three programs that can create and edit Acidized stretchable files that follow tempo changes automatically: Magix Acid Pro, Magix Sound Forge, and Cakewalk by BandLab – the latter is a free download, although it’s Windows-only, and works only with 64-bit computer systems.

Go to BandLab.com, register, go to Apps, and download Cakewalk by BandLab. Follow the documentation for information on how to create and edit stretchable audio files. Note that most DAWs recognize these files, and will stretch them to fit tempo changes (and sometimes, pitch changes as well).