Fair question PY, there is no reason IMO for most unit addresses not to be available. "Tracing" or whatever the old postie service called it often did a good job of getting mail to the addressee, but no longer it seems.
For letters not packages, try RHQ or corps HQ if you get no better suggestions. I have done this successfully with a covering note, enclosing stamped letter for forwarding. Of course you need to find the HQ first! Often available from the regiment's or Corps' website.
If you know the UIN of the unit try 0141 224 3600 (Enquiry service Glasgow).
They hold all unit addresses and should be able to provide it to you no problem.
otherwise contact Disclosures at Glasgow and they might be able to help.