spa to avi conversion can potentially mean several things as different kinds of .spa files exist. Considering this is sought as export to AVi, it might be actually related to export of old videos from a tool called FutureSplash Animator, which actually was predecessor of Macromedia and later Adobe Flash.
To be perfectly honest, it is unlikely you can convert these files using modern software and unless you have access to the original tool or perhaps earliest versions of Macromedia Flash, there is no way how to do it.
We do not know where to find FutureSplash Animator, but we know you can still find Macromedia Flash on abandonware websites. Hope this helps.
As for other .spa files, we do not know about any that can be converted to AVI directly. Bitmaps from MikuMikuDance can be perhaps used for textures for 3D animations that can then be exported to AVI format. But that seems to be a bit far-fetched.
Updated: September 22, 2023
