Each coliseum has a slave market.
The londinium one is the cheapest one. The others are much more expensive.
Also, you should buy most of your slaves from londinium.
When you buy a slave, you MUST have in mind what type of gladiator you want to build
Then decide which to buy according to :
- His/her sex (male have a slight strenght bonus & females have a slight charisma bonus)
- Required mp level of the glad you wish to build
Also keep in mind that mp will rise when you train them. Check our article about mp to start how to limit this or on the contrary to farm MP
Tip:
If you want a 760 mp glad he must have 730 MP + 30 MP bonus from gear. buy a 720 or a 730 MP one in London. He will be 730 by level 70.
If there are none, you can renew the londinium coliseum by doing: SAVE - BACK - BACK - YES - START
With other colisuems, if you wish to refresh the slave market just refresh the game.