What is this bolt called? It's part of my front axle and I need another one like this to keep the wheel on. Links to where I could buy one would be appreciated too.

The bike is a fatbike with suspension.

Bolt threads into this barrel which looks the same on both ends which becomes the axle. The screw itself is m10x1.25 with a fat allen key insert.
Searching for "Large head socket screw" gets close, but no cigar.
Also try 'cap screw'.

You need to know the thread, of course.

You may be able to replace it with a (rounded) socket button screw, eg. https://www.kayfast.co.uk/category/517/Socket-Screws

Update: Search Socket Pipe Plug, also described as Flange Plug and Blanking Plug - these look very similar:
https://www.accu.co.uk/en/flat-head-socket-pipe-plugs/177868-HPPF-M10-1-A4-BL

Or: Slotted blanking screw eg. https://www.ebay.co.uk/itm/NiceCNC-Billet-Mirror-Hole-Blanking-Plugs-M10x1-25-Aluminum-Motorcycle-Universal-/303185376819 - interesting that these are also available with a left-hand thread.

You need not (necessarily) be wedded to the allen socket - an ordinary M10 bolt over the end, perhaps with a suitable washer, might suffice, for quick replacement.