Keep in mind that what you are really trying to do is stop the IR emitter lines from being used as antennas, either transmitting *or* receiving. So, ideally you would want to filter (attenuate) those frequencies that the MRF responds to so that it won't pick up noise or reflected signals on the IR lines that block out the real signal coming in through on the antenna.

Unfortunately there is almost no way to know if the cores you are buying will help filter out the frequency range you need, unless the seller provides specs or a model number so you could look it up. At least they will probably help clean up some of the extraneous noise, which should help. They filter out a wide band, not a narrow one, so that makes it more likely they will work for your application.

I've had very good luck buying batches on eBay so far. I don't remember exactly how much they were, but $1 each seems high. You may want to poke around on there a bit to get a feel for the prices and sellers before buying.