Where can I find a RS1b bass driver?

Try the Infinity group over at Yahoo.

I've bought a lot of parts(cones/surrounds) from Millersound. A very knowledgeable and pleasant person to deal with! He would be my recommendation as well.

Just heard a fabulous SOTA pair this week!Still a fantastic speaker,totally competitive with the best of today's big boys!!
I'd second contacting Bill Legall.
If he can't help,don't stop!!!A great and timeless design,IMO!!!
You may want to consider replacing all the bass drivers with current drivers.I'm sure there are those out there capable of aiding you,here.I know of a fellow,Lewis Mansky,of NYC who could probably help here.A great tweaker/specialist.I'M SURE THERE ARE OTHERS.
I'd check with Bill first.

Contact Bill Legall of Millersound, (215) 412-7700.

Best loudspeaker repair guy in the business. Infinity woofer repairs, particularly the IRS line and RS1-B, are what he is renowned for. He'll be able to restore them without issue.

Contact Watkins Stereo and Engineering the developer of many of the Infinity woofers they also do speaker repair

Check on E-Bay.

Good luck!!!

Rick (RWD) A very happy RS1-B owner.