Just do a search on ebay or amazon for "soda stream adaptor hose" and "20# CO2 bottle" or something similar. I think I ended up getting the hose off Ebay because they had a wire braid covered one that I liked and the bottle off Amazon.

You can get the CO2 filled at any welding supply company where you would buy acetylene. You can also just rent a bottle from them (but I found for the amount of gas we would use buying the bottle was better). We got it filled once and just now it is getting low enough so it is barely popping off the pressure valve on the soda machine. So it is getting low and will need to be refilled some time this summer. I think it has been a couple years now. Seems like it was around $25 to get it filled. There is a definite up front cost but once the initial cost is absorbed, it costs almost nothing to operate. Maybe 12 dollars a year and we are home about 7 months out of the year.

I bought extra bottles and charge 4 or 5 at a time (we have 5 bottles total). As we use them we refill with reverse osmosis tap water and refrigerate (we happen to have a second small refrigerator in by the soda machine). Chilled water takes on CO2 better than room temp water though both will work. Then when I get 4 of the tap water bottles I charge them all at once.  That reduces the CO2 waste as you should shut the valve off at the tank and discharge the hose pressure after use. Otherwise could cause a hose leak or rupture over time and lose the whole 20# of CO2. Takes a couple minutes to charge 4 liters of soda water. We use 4-5 liters of it a day.

Here are examples. Not necessarily the exact ones I bought, just what I found on a quick search.

https://www.amazon.com/Victory-Aluminum-Cylinder-CGA320-Valve/dp/B07BDFLSG4/ref=sr_1_2?crid=WPUG1G3XYV2T&dchild=1&keywords=co2+tank&qid=1620576671&sprefix=co2%2Caps%2C233&sr=8-2

https://www.amazon.com/Cartridges-Connector-High-Pressure-Compatible-SodaStream/dp/B08LQQKKD3/ref=bmx_5?pd_rd_w=m1lxY&pf_rd_p=b56a886c-2bb4-4e74-b4cf-23d7a76693c8&pf_rd_r=CSETE3QZFKY5C24FVHQX&pd_rd_r=62e03e7d-bd4f-4870-89b2-f182d22d32da&pd_rd_wg=7cgSg&pd_rd_i=B08LQQKKD3&psc=1

https://www.amazon.com/Original-Sodastream-Kidscare-Extendable-Cleaning/dp/B07BR3H9WP/ref=sr_1_9?crid=U3NQJNCQW50S&dchild=1&keywords=soda+stream+bottles&qid=1620577122&s=home-garden&sprefix=soda+stream+bo%2Cgarden%2C195&sr=1-9