Linker for what? There are some linkers that pull double duty via things like littlewriter but it is not a sure thing.
Equally the linkers themselves can be quite desirable these days (if yours got lost or broken...) and you could probably flip it and fund another flash cart.
It was never going to be like buying a few slightly specialist SD cards and carrying on with life if that is what you were hoping for, give or take what I cover in a second.
If you want blank cards for homebrew then most just use the EZ3 in 1 and program them via the DS. GBA sized ones do exist but they are quite pricey and hard to come by as others already rinsed the supply. If you want to mod a DS lite sized one to fit in a case then that is an option. Some also open them up and snip the write enable pin to prevent others from overwriting them.
If you fancy going a bit more DIY there are options too
http://reinerziegler.de/GBA/gba.htm