For just playing then most people would say get a flash cart. Main options there being Everdrive, EZFlash Omega, EZFlash 4 aka EZ4 and if you have a DS and DS flash cart then you might also consider an EZFlash 3 in 1. You have some scope for emulators as well on the DS, though GBA flash carts are the better option in many ways.
If you want to have something to sit on a shelf and be a normal game otherwise then that would be heading into the reproduction aka repro territory. This is a very contentious issue around here; hackers are usually not fans of it as people try to sell them, and said people selling them tend to use bad/old versions of the hack, and selling it as well as out and out commercial piracy, said paid piracy also leaves a sour taste in the mouths of many for obvious reasons and may also attract attention from legal types we don't care to have the attention of. As emulators and flash carts were readily available for anybody that wanted to play one then most are OK with potentially offending the "I want one for me, not to sell" crowd.
Chinese efforts right now are producing some fairly quality repros that you have to know what you are looking at to tell (
https://gbatemp.net/threads/identifying-fake-gba-games-a-useful-faq.327354/page-9 ) but I have no line, not that I would give it should I have it, on something you can send a ROM and payment to, and in return get sent a stack of games.
A few years back the 3 in 1 mentioned above came in a GBA slot form factor (as opposed to the more common today DS lite size form factor). It was then popular for a while to get said 3 in 1, burn the single game to it and play that, some also lifted pins to prevent overwrites. Said popularity means it is very hard today to find a GBA size 3 in 1, and said 3 in 1 also has a battery to manage the saves so you have to look after that as well. You could use a lite sized one and stash it in a donor case from whatever GBA game you want to scrap (or otherwise build a case), indeed many did just that.
There are homebrew designs of flash carts (
https://web.archive.org/web/20180426222911/http://reinerziegler.de/GBA/gba.htm#homebrew flash cart ) but they are crude and lack support for saves. You could adapt one of those to saves and otherwise be better but that would need some electronics skills which you probably don't have enough of if you are asking questions like this.
So between no availability and serious downsides your main options are get a flash cart, or use an emulator.