Hi foxano,
There may be two ways to get more accesspoints on msm760.
1. Buy another msm760 and team them together. The controllers share licenses so you could get 80x accesspoints, BUT I am not sure if the default 40x ap licenses are shared. Just remember you will loose all over 40x if one controllers dies.
2. Search Google for "buy hp msm licences". I found some places with "In Stock", so you may be lucky.
I have 5x MSM760 controllers with 500x MSM460/560 which we are slowly moving over to Aruba APs (325s). They have built in controllers which work very well. So I have one "controller" per building at the moment, and was way easier than MSM controller to implement.
Regards,
Craig.