You have a couple of choices depending on how quickly you need it, because some Janome brand parts and accessories are on backorder due to the pandemic. There are many generic versions available, but IMHO they aren't as durable as the Janome brand. Some work better than others.

Note: these links are provided solely for  part number ID purposes for web searching, not for recommending a specific vendor.
With Janome, 
You can buy a plain open toe walking foot, https://www.sewingpartsonline.com/elna-walking-even-feed-foot.aspx
Or, you can purchase their "even feed" foot set, which is a walking foot that has interchangeable feet, open toe and regular, plus 2 attachable guides. https://www.sewingpartsonline.com/low-shank-convertible-even-feed-foot-set-janome-214517004.aspx

An example of a generic foot: https://www.sewingpartsonline.com/walking-foot-elna-janome-214874013.aspx?
While it looks very much like the Janome, it doesn't hold up to long-term use for quilting and the feed dogs sometimes fail.
Hope this helps.

Jan