For power management and other secondary parts, I would recommend
https://4layertech.com/ , and depending on your build if you want hdmi out or other more advanced features then I would also suggest looking at
https://electron-shepherd.com/store/
And for batteries, looking at this guide:
Where to buy would depend on where you live, searching for 18650 Li-Ion will often give results as they are rather common now a day, otherwise, you can look at for instance amazon or ebay for rechargeable 3.7v batteries.
For cases, you can try to buy a premade case, 3d print your own (or find a friend who can print one for you, or find a service that can 3d print for you at cost), or frankencase (Hack and glue together various cases to create your own)
Here is another guide that goes more into cases.
You can look at the model repository here on the forum and see if for premade cases that just need printing if you want
Here is the link for the g-wii ready to print and supposed to be easy for beginners to get into
Hope this helps
Edit
Lol, I fell for it again, answering an old post that got bumped, without checking if the op was still active and if it was still relevant.
