For the purpose of backing up a card to play it on an emulator you are fine with even a cheap and simple backup unit.
My first machine was a Super Pro Fighter from CCL with just 16 megabits of memory to store a game, but I was able to dump even the biggest and newest games onto up to 4 1.6 meg disk and then join them together with a programm like ucon64 to get an image that worked in Zsnes.
The problem starts with wanting to play the games from disk. First of all the memory of the machine must be big enough to hold the game. With 32 megabits your good for every game but Tales of Phantasia (it's 6 megabytes). Then it depends on the additional chips inside the copier. SuperFX games won't run on most machines. I never got them to run on my Super WildCard DX 32. DSP games like to loockup or don't even run at all. Some games even sport a more or less powerful protextion against being used in a backup unit. While Killer Instinct runs perfectly from disk on my WildCard it displays a "don't play this on a copier, copiers are illegal" message when play directly from the card through the WildCard.
So if you just want to backup your fine, if you want to play the backups on the copier you might run into bigger problems.
Try your local flea markets or second hand gaming stores for backups sytsems. I'm quite sure that they got banned from eBay, but you can try. I bet you will still find some units on Hong Kong markets so there might be some import sites on the net who cary a limited amount.
You better check the list on the page RoushiMSX provided when you consider to buy a specific copier. I guess it is no shame to spend more than 100$ on a WildCard DX or DX2 maybe even 200$ but some copiers (especially the early 90s ones) aren't really worth that kind of money compared to the features they provide.
By the way does anybody know if the WildCard DX2 32 can be upgraded to 64 Mb? I might consider it if it ain't to exensive.