Mace is not that easy to find...but you can find it a gun shops and gun shows. Pepper spray is the 'weapon of choice' and that is EASILY found at any drug store chain or Sams Club possibly. I carry one in my purse, and you probably could keep in your tool bag. They are no bigger than a small cigar.
They can clip on your key ring...and come in all sorts of ways: a clip to carry it on your person, on a necklace (one of those convention types with the logo's on them). If that will give you some 'more' peace of mind, by all means, make the $9.00 purchase. Also, Pepper Spray (unlike Mace) does not expire....so you can keep it for any length of time. They also sell larger home/car ones...bigger like a can of hair spray.

Of course, laws vary in each state. You can't just 'spray' someone because they give you the creeps.

I'd also take a class in self-defence...as many have suggested. You can do this at community colleges or at a local Karate/Kung Fu/Ti Kwon do dojo. They're all in your yellow pages. Heck, even the YWCA has some.

Good luck and be 'viligant'. I'm not sure where you are located, but growing up in Los Angeles...I got 'street smarts' FAST being in a big city and taking public transportation most of my teen life.

I hate to say it, but you do need to be CAUTIOUS in this day and age...it could save your life.