You can download all known kickstart ROMs here, alternatively there are several ways of getting them legally, speaking of legally, laws vary from country to country and which licence agreements are applicable, whether there are “fair use” laws, or not so I’m not going to comment on this and I’m not recommending or advising anything in particular, please do your own research.
Cloanto / Amiga Forever / Windows
The Amiga Forever package for Windows is great, choose the package that suits you and it includes different (classic) Amiga ROMs, check the licence to see if you can do what you want with the ROMs you get in the package.
Cloanto / Amiga Forever / Android
If you’re running on Android, the Amiga Forever Essentials might be a good option for you, I suspect you can (on a technical level) copy the ROMs from the package, I don’t have this so I have no experience, but (apparently) it comes with a bunch of ROMs, I suspect it’s not an unlimited licence, perhaps only permission to use on an Android emulator? but take a look it might be the cheapest way just to get the ROMs legally.
Hyperion
Hyperion are updating and developing new features for classic Amiga hardware, 3.1.4/3.1.4.1/3.2/3.2.1/3.2.2 these are designed for Amigas with hard drives and expanded memory and so are not a good option for unexpanded A500’s
eBay, Marketplace, Craig’s List, car boots
There’s loads of Amiga’s kicking around, lots of used kickstarts on eBay etc. if you own a physical kickstart, it’s yours, again, I can’t comment on the legality of the use of the software on it, whether that’s yours to do whatever you will with it or not.
Ripping your already owned ROMs
There’s a few ways to grab an image from something you own into a file.
#1 Use a EPROM reader
A cheap option is a TL866 eprom burner, it can read and write your own kickstarts, you will need an adapter to use it with 27C400 style ROMs, you’ll need to physically unplug the ROM chip to read it
#2 Use Amiga code
You can install some code on your Amiga such as GrabKick and this allows you to read the ROM from memory without having to dismantle or unplug anything
#3 Use the WinUAE debugger
The debugger in WinUAE lets you save chunks of memory to a file, this is useful for encrypted ROM files or kickstart boot disks where the file is hard to extract, for example;
<shift+F12> # To start the debugger once your WinUAE Amiga is booted
S C:\Temp\rom.bin 0xF80000 0x080000
#4 Use a ROM decrypter/loader
There’s a few bits code that can read different format ROM file, Cap should be able to read any ROM format and save the raw ROM file, but other software exists, google is your friend.