Your question deserves attention. Because there are enormous variations between different locations as far the prevalence of fake boobs is concerned.
Touristic spots see a lot of traffic so there is a good probability that some large breasted women will show pretty often.
Big metropolitan cities also have a higher probability than smaller cities.
Basically, there are several factors that will affect the probability: culture, economy, population, traffic and environment:
1. Culture: some countries/regions have a higher prevalence that is linked to the local culture, for example Venezuela, Colombia, Brazil, California, etc.
2. Economy: Apart from exceptions like Venezuela and a few others, it is more common to see fake boobs in richer countries. In poorer countries fewer women can afford it, and there may not even be clinics that offer the procedure.
3. Population: let's suppose there is 2% fake boobs in a city of 1.000.000 and the same percentage in a city of 10.000. That makes 20.000 pairs of fake boobs in the big city and just 200 pairs in the small city. My guess is that you will more frequently spot them in the bigger city.
4. Traffic: a place with lots of traffic also increases your chances.
5. Environment: sea side locations, beaches, parks, etc. gives you better chances of spotting big breasts. A rich nightlife too.
I am actually asking myself the same question for the prevalence of muscular / fitness women. And there are similar factors affecting the probability of finding such women.