you are entitled to install 2015 sp5, but now since of subs you can't download the files.
Call your VAR and ask them if they could share the files with you -it shouldn't be a big deal.
You can't download anything from the website if you're off subscription, but I believe you can get service packs directly from your VAR if they were released while you were on subscription. However, I'm pretty sure that SW2015 sp 5 wasn't released until after SW2016 was released, so you're probably out of luck on getting it.
I have just noticed that you did contact your VAR.
regards from SolidSolutions
Thanks for the suggestions (and thanks to Solid Soloutions for providing the link, much appreciated).