You can learn some things alone. But there's a lot of caveats worth noting.
Self defense is risk reduction
Martial arts can give you some tools, but it's worth noting that the best safety is avoiding danger where you can.
So - the best training you can do alone right now is finding away to get yourself up on some short distance sprint training. Obviously, if your neighborhood isn't safe, it may make that difficult - but running IS a great skill to have trained.
Also realize that getting robbed is unpleasant, but not the real problem - it's people trying to hurt you that you really have to worry about. It's not worth getting into fights if someone is going to take your money.
Things you can learn without a teacher
Do you have a friend or a family member who would be willing to practice with you? That helps.
The thing you can generally train is how to push people. Push them with your hands, push them with your forearms, push with a shoulder check, push with your hips. If you can shove someone and run, you have a simple skill you can use.
You can look up stuff online like Tony Blauer's S.P.E.A.R. defense videos - he uses a simple move that works well for that kind of thing.
If you don't have a friend to work with... well, you can try practicing your shoves against things like poles or indoor building supports or the frame of a doorway. That said, all of this basically boils down to shove and run.
A simple tool
The last thing, is you can get yourself a small, heavy flashlight. A 5-7 inch Maglite usually is small enough to go in a pocket, heavy enough you can get an effect from it using a hammer fist (grip the flashlight, hammer it down on someone) and they're usually not more than $20 tops, making it more affordable.
Again, the goal of using this would be a stun and run - hit them, push them, run.
There might be more people in your area who want to train
What a lot of folks don't realize is that there usually is a fair number of folks who want to learn self defense, especially if the area is not great. I would definitely do some searching online - craigslist, meetup.com, etc. (or the equivalents if you're not in the US) to see if you can find a study group in your area. You might want to ask around at places like a YMCA, any churches/temples/mosques and if there's any local community colleges to see if anyone has leads as well.
Many of these groups charge nothing, or a very minimal fee compared to martial arts instructors or schools. I know my first serious teacher would teach women and youth for free and we trained in a church basement for many years.