Double joints on VOLKS SD10 and SD13 elbows?

Yes, AFAIK all SD10 and SD13 releases have them these days. Even more recently, they've introduced double-jointed knees too (at least for the flat feet). I'm actually not sure exactly when the double-jointed arms were introduced, but it was sometime during the availability of the standard models Saki, Coco, and Chiyo. I remember that those standards came with single joints, but the double joints became available for FCS and other releases before they were discontinued. Then the next round of standard models came with double-jointed arms. I believe the current standards have both double-jointed arms and legs.
Edit - forgot to address the question about buying arms separately. I don't think Volks sells individual parts other than the hands and feet. The bodies with double-jointed arms are pretty widely available now on the secondhand market and not very expensive, so I usually just buy a whole body to upgrade my single-jointed dolls.

 

Double jointed elbows are standard now! You could order the single jointed ones in FCS if you wanted, but it's fairly safe to assume SD10/13 will come with double jointed elbows. You might be able to upgrade your old girl's body to one that still matches close enough, but you're unlikely to find the arms separately.

 

I think Mai, Nico, and Mel were the first Standards with double-jointed elbows, and they were released in 2017.

 

DP32 (2014) Lieselotte and Charlotte come with double jointed arms SD-A-03. I'm not sure if that's early enough to find a resin match for your older girl, but that'd be a little closer than a 2017 standard. FCS would likely have offered them earlier than the 2017 standards as well.