Probably the cheapest flash card is here:
http://www.aliexpress.com/item/supe...DS-FC-GB-PCE-SMS-can-download/1630903734.html
Also, its one of the best flash carts! I don't own it, but from the feedback, it has great ratings!
I do not know where you are getting your ratings but the supercard GBA family is probably the worst of GBA flash carts, in as much as basically every other card will run the game if it fits where the supercards will struggle with a lot and have slowdown in more. You can do a fair bit with the GBA library on them, supporting SD means you can fit a lot of said library on at once and it has some interesting features but it still but it stops considerably short of good if you compare it to its peers.
As for the OP I do not know which vendors are doing what for GBA flash carts, and especially not in Australia (or somewhere that has cheap prices and free shipping). Most GBA carts should link games between stock, each other, other flash carts and whatever other combo though you may have to fiddle a bit (nothing major). If you actually want to run one on a GBA (the DS does not have link support for anything) then you will probably want a cart you can manage from the GBA or PC, this eliminates the 3 in 1 expansion pack and other expansion packs that people favour for DS use.
This means your main options, barring finding something very old or very rare are
The 3 in 1 which we just discounted as it wants a DS to flash/fiddle with. You can get one game at a time on there though (and return it to the DS to change, including keeping the saves) and that is enough for some.
The supercard GBA stuff. Supercards never did well at GBA (they skimped on memory and suffered because of it, their DS support was long lived and very good at the time though) which is why you can still find them today.
The EZ4. A new batch was made last year (and possibly are still made) so they are available again. The normal version will need miniSD, the others (save for the lite compact) will need microSD (2 gigs or under) and some modding to fit in a standard size GBA slot. The EZ4 lite deluxe has some larger memory sections which can make some things a bit easier (mainly running those small handful of games that went above 16 megs, which you can do on the other models but it is annoying if you want to swap all the time).
A fire card/fire card clone. These are older style GBA flash carts (they tend to come in 16 or 32 megabyte flavours, 32 being quite rare), are flashed using a PC and if it fits it can probably be made to run. I have not seen many this last year or so but when people find one tucked away somewhere it is probably one of these, or an expansion pack aimed at the DS which we already covered.
Note do not get a supercard rumble, an EZ4 lite compact or a M3 professional. All were aimed at people wanting DS games back in the early DS days, they lack the hardware to play bigger GBA games (or any in the case of the SC rumble). As they do not play GBA very well and nobody uses GBA slot cards for DS games (well I do but that is a different story) then you can still find these in some places.
Again though I have no idea on what is going to be cheap for you to get in Australia. We have stepped back from the events of last year where it was buy it wherever you can find it (the EZ4 people came back and did another run).
By the way you can use link on emulators, specifically VBA-link and I think VBA-m these days too.
tho' i don't know, don't go by what I think, haven't bought there.
