Hi Michael :wave:
Unfortunately, $100 isn't going to get much for you in the way of a piano ... either electronic or acoustic ... the cheapest Casio electronics are several hundred dollars to start with.
One way to start is to rent a piano from a dealer ... most piano dealers have a rental program, others have a rental with option to buy program ... but that is certainly going to chew up your $100 pretty quickly.
Another way is to check with local churches in your area ... ask if you can practice on their piano when the church is not in use for services ... most will let you do this without you having to pay, but some may suggest a 'donation' of some sort. Check also with local universities/schools - even music stores that have practice rooms are another possibility.
I also heartily recommend securing the professional services of a teacher/tutor ... paying for lessons was not in my 'budget' for certain, but I made the sacrifice and got a teacher anyway, a decision I have never regretted as I have been a professional classical organist for the past 52 years.
As for a cheap piano, as mentioned try the charity/thrift shops ... you may have to pay for moving it, but the prices should be within your present budget. If the piano is horridly out of tune (as most charity/thrift shop pianos are) it is going to cost $45 to $75 for a good tuning.
Keep us informed about your progress on this ...