@Frank Bonzai, Costar has multiple products, you likely need to call and talk to someone. Expensive.
Listsource does not offer phone numbers.
What didn't work for you with ProspectNow?
I just did a quick search and found 257 properties zoned apartment, with 16-50 units in the City of Las Vegas. As is typical, most are held in LLC or partnership names, which makes phone numbers very difficult without some additional work to find the name of the principal of that LLC, and then their number. Looking at our data we only had phone numbers for 11 of the 257 for that reason, whereas for single family the match rate is typically 70-80%.
One thing to keep in mind is that any service provider is only going to be as good as the underlying data. A lot of investors looking for multi-family run into problems not because of the service provider, but because the data at the county is poor to start with. That they had 257 properties coded both as apartments, and with units data shows that Vegas is better than most counties in populating this data, but you may still find that is an incomplete list. There are ways around that when it happens - basically compare some apartments that you are familiar with, against the county data, and find criteria that is unique to those properties that you can use to find all of them. For example in one county they didn't record units or beds on apartments, but they did record baths. So I was able to use baths as a proxy for units to find all the apartments of a certain size.
Hope that helps.