Is it based on a completely random factor?

The manual says if i spend more than $5,000 buying drugs from a dealer.
It says i can get a bitch this way from a city only once. and can do this 120 times in one day.

Ive been to like over 20 cities today and only received 3 or 4 bitches. Seems impossible to make 120 in one day. Whats the best way to ALWAYS get bitches when i go to a city?

Is there a specific amount i have to spend in each city? or
Is there a specific drug i have to buy from each city?

Please help. Thanks.