That part is called a torque rod.
I haven't heard that they're on back order. Phone your local dealership, give them your reg and tell them you need the upper torque rod (the one near the coolant reservoir). See what they say.
Your mechanic could be right, but I doubt Volvo would have these on back order. Its such a common part to fail that normally they're always available. Then again, the demand could be so high that they've ran out!
You can also get them from Parts for Volvo website. I don't know what brand this particular one is (its not genuine, as genuine are dearer). You'd have to check with them what brand this part is. If its a good brand like Febi, then its worth buying. Interestingly, Febi make torque mounts for Volvo. I bought a "genuine" torque rod from Volvo and it had a Febi stamp on it. Enquire what brand this one is. You don't want a cheap junk one either. It has to be a reputable aftermarket brand, otherwise it'll just fail prematurely and you'll be buying another one. Buy quality and replace once. Buy cheap, replace twice (or more)! Quality is your best option.
https://www.partsforvolvosonline.com...e88ae0c6053f9b