Go to your local hardware store. I like Ace (but Home Depot works too). They may not have tubing listed in millimeters, so I'd just take the intake/output pipes with you and grab the tube that fits must snuggly. You can pick up some hose clamps while you are there.
Shouldn't cost more than 10 bucks for as much tube as you need and 4 hose clamps. Maybe closer to 5 bucks.
I always buy my vinyl tubing from Home Depot or Lowe's. In fact, I just purchased 10' of 7/8 OD x 5/8 ID tubing last night to redo the plumbing of my Rena xp1.
If you go this route, I would buy the stainless steel hose clamps that you tighten with a nut driver. Also, if you get a tubing that is very tight, you can always boils some water to soak the ends of the tubing in. This will make the vinyl more pliable and easier to work with. This method has worked for me for decades.
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