I have only worked on 2-3 Timex watches, so I am not an authority, but they are simply made & I believe the M24 was a fairly common Timex movement that likely shares parts with other movements. I do not have a parts listing, so I cannot verify the correct mainspring for your movement or how available that & other commonly replaceable parts are.
In most cases, as long as your movement is complete (ie, not missing any parts), any watchmaker should be able to overhaul it, which will either put it into functional condition or give you an idea of what part or parts need to be replaced to get it there. That way you have a specific goal, rather than just a general assumption that parts are difficult to source. I would begin by checking with a local watchmaker to see if they can assist you. Otherwise, post a request for help in Timezone's Watchmaking forum. You might contact this seller to see if they have the mainspring you need.
Worst case, you can always get a generic mainspring to replace a worn 1. Either have a watchmaker handle the work, or, if you have some watchmaking skills, remove your spring from its barrel, measure the length, width & thickness of your spring & look for a match (with similar ends/fittings) on any of the watch parts houses like ofrei, cousins, esslinger, etc.