There are two ways to get upgrade points.
1) Do the Main quests. If you do the quests for the mage's guild, you get points. about 10-15 per, I believe.
2) Sell slaves. The better the slave's rank and stats, the more money you get. But you also get upgrade points for high rank slaves.
These are the only ways to get them. It's a little grind-y to get a decent number of 'em, but given time, you can get as many as you need.