battery packs - plural 
for Elektron boxes:
https://smile.amazon.de/gp/product/B07Q29BGNG/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
for Zoom H6:
https://smile.amazon.de/gp/product/B00EUUZNJE/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
the DC 2.5mm to 2.1mm cables:
https://www.aliexpress.com/item/32771829446.html?spm=a2g0s.12269583.0.0.112b3d07V8PGJh
here a detailed look with specialiced velcro fastener (hope this word is correct) 
https://smile.amazon.de/gp/product/B000VDB4NI/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1