I go to a company in my area that specializes in home brewing. I bought the tank there and just exchange it when it runs out.
I don't refill any small tanks. I just hook the 20lb co2 tank to the sodastream with this....
GURUDAR CO2 Tank CO2 Cylinder... https://www.amazon.com/dp/B086XH2FTK?ref=ppx_pop_mob_ap_share
I've tried two different ones from amazon. Both have a bad taste and smell.
https://smile.amazon.com/gp/product/B086XH2FTK/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
https://smile.amazon.com/gp/product/B07MKN2HP4/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1
Also interested in one that works.
I bought this adapter for my 5 lb. cylinders (2). Works great!
https://www.amazon.com/gp/product/B086XH2FTK/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
A valve isn't needed if you do it this way. Some people turn the tank off when it isn't being used, which I did for a while, but now I just leave it on. It's a SS Teflon lined hose that has a quick connect fitting on the Sodastream end.
It's this one: https://www.amazon.com/dp/B086XH2FTK
I got this one: https://www.amazon.com/dp/B086XH2FTK?psc=1&ref=ppx_yo2ov_dt_b_product_details
I can't vouch for it long term, but reviews are good and I had no issues at all installing it.