My car stereo is very low volume and I listen to a lot of live music. I know nothing about cars and am looking for a way to get louder speakers without having to take anything apart. Is there any kind of system that I can plug into a USB/AUX/cigarette port that I can sync up with my existing car speakers? (Probably not but I thought i'd ask.) Or does anyone have suggestions of any portable speaker that I could put in my car that would still sound good if I was using that instead of my existing car speakers?
The speakers aren't going to be your problem; putting higher powered speakers in won't actually make them any louder (it just allows them to handle more powerful inputs). Putting more sensitive speakers in would work to a point, but only if you know the sensitivity of the existing speakers, and can be expensive for a minimal improvement.
Ideally, you should take the car to a car stereo store, and let them know what you want from the system; they'll recommend a head unit, amplifier and speakers that all work well together. It's the best solution, but not the cheapest.
If you want a cheap solution, replace the head unit currently in the car, and keep the existing speakers. For about $100, you should get a worthwhile upgrade that can have an amplifier added at a future date if it still doesn't give you the volume and quality that you want.
The tl;dr; answer? No.
While there are potential external speaker options, they will not sync up to your existing stereo, so issues of placement would probably negate any of the gains from them. None of the small bluetooth speakers which would actually fit on your dash are going to provide any clarity at the volumes you need to overcome road noise while driving. You could theoretically grab a pair of small speakers and a 50x2 12VDC digital amplifier, figure out where to put them, drive around, reposition them, reposition them, reposition them... or do it right (and safe) and upgrade your head unit.
If you DIY it - most vehicles with "not loud enough" stereos aren't bad to DIY - you could get a discless head unit for under $50, a mount kit (if needed) for another $10-$15 and a wiring harness for another $10. After some minor disassembly and a little soldering or crimping, you'll have a head unit with enough output power to hear your music and support for digital files or bluetooth audio.
Spend a little more on the head unit to get fun features like a high pass crossover to avoid wasting amplifier power on low frequencies that will detract from your music and/or damage your factory speakers, and additional amplifier outputs to add subs or external amplifiers if you want cleaner power (and want to upgrade the speakers so you can hear the difference).
Car audio is certainly not an all-or-nothing game. You can start small/cheap and only go further when you decide you want more. There are plenty of people out there who are happy to guide you on that path. Loud is cheap, loud and clean will cost a bit more, and flat is where it gets expensive.
There are systems that are cigarette/aux/usb/bluetooth that fit your needs as posted.
For example look at Sondpex Bluetooth Speaker System & Digital Music Player CSF-E65B.
It only has the drawback of taking space inside the car.
You might find this article interesting and it has several other suggestions on wireless car audio. Wireless car audio