The cheap Chinese blanks from eBay tend to be a bit hit and miss, especially the smaller (6-8mm dia.) sizes. I have found that if you type 'huge ferrocerium rod' the search brings up a load from the Philippines, some are 5" × 1/2", others 130mm x 13mm but they all seem decent. Technique is important so my opinion is that longer lasting means you can practice with it and not be concerned about wearing it out. They are probably not LMF standard, but harder so will last longer. Also the larger sizes tend to be more versatile, lightweight tinder can be pinned down as well as using it to throw sparks at fluffy tinder.
At around £3.50 you can't really go wrong either. If it's crap just keep it for lighting gas stoves.
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