Checked with their site and this should answer you question. Found it under their FAQs.

Can I purchase a shaft directly from ALDILA?

Unfortunately ALDILA does not sell directly to consumers, but our shafts can be ordered and installed by any of our authorized dealers. Please visit our dealer locator or call us at (858) 513-1801 and we can help steer you in the right direction on whom to contact.

If the R75 shaft from 13 years ago is still an option. Adila only lists an NV iron shaft.

I’ve never had any luck buying a single iron shaft. Both locally and online. I was advised I needed to buy a set of shafts. So even a comparable single shaft might be an issue. You might have to find a replacement by buying a different 4 iron (unlikely to be an 712 AP2 iron) and pulling the shaft.

Lots of golf repair shops near you in Texas. They can order you a shaft and even do installation if needed. They will work with you for a comparable shaft the one you want is not available.

I would consider ordering a new one assuming the shaft is available on (or off) menu at Titleist. You can call customer service and ask.

The reason I would consider ordering a new club is that first any change in the weight and balance point of the new shaft compared to your old shaft could throw your swing-weight off dramatically! Thus, if you order a new club all those factors will be fixed...

You can do this through Titleist.com or by ordering through your Authorized Titleist Dealer.

Cheers!