I have seen them on ebay quite often, but you'll probably have to buy the speakers to get the drivers (I am a long-time Fried devotee and the Q5s are fine speakers). BTW, the Betas and the Q5s went out of production (to my knowledge) in the 90s.
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Yindi of Shanghai, China, known in the West as (Shanghai) Silver Flute manufactured the drivers you mentioned. As is the normal custom in the industry (despite claims to the contrary by most loudspeaker manufacturers), they were an off the shelf item, as opposed to something proprietary for this application. They're an excellent replacement in general, and a true bargain in raw drivers.
You can purchase both at Madisound, at quite modest cost http://www.madisoundspeakerstore.com/silver-flute-woofers-6-7/silver-flute-w17rc38-08-6-1/2-wool-cone/ and http://www.madisoundspeakerstore.com/approx-8-woofers/silver-flute-w20rc38-08-8-wool-cone/
The foam surrounds on most Betas rotted out long ago. If you happen to have Gefco (black paper w/black butyl surrounds) drivers in your speakers, you might want to have them rebuilt instead of replaced, as they're quite nice. Bill Legall at Millersound would be best at that http://millersound.net/service.htm Usually, they need new voice coils, but Bill does the spiders at the same time. So, you wind up with "new" drivers when he's through with them.