The IBM PCjr has a dedicated community of enthusiasts who have created invaluable resources for restoration and upgrades.
Mike Brutman’s PCjr Page
Mike Brutman’s comprehensive PCjr resource is the absolute authority on all things PCjr. The site features active community forums and detailed technical content covering everything from basic setup to advanced modifications.
Resource: https://www.brutman.com/PCjr/pcjr.html
TexElec
TexElec manufactures specialized boards for the PCjr including the JRIDE (Jr IDE Board), which allows you to add IDE storage to your PCjr. The owner is responsive and proactive with customer support.
Resource: https://texelec.com/blog/