Questions about a quadplex and running numbers

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Hey guys — so I'm wanting to run numbers on the purchase of a quadplex somewhere in the Houston area. 

1) Where would you look for the property?  MLS, LoopNet, a broker, etc

2) How do I find out what rents are going for in certain area (aka how do I find out what each unit of the quadplex will rent for) 

Also — when looking at some listings, sometime the broker will say that the units are renting for a certain price... should I trust this number or should I ask for proof? Is it rude to ask for proof? I don't want to overstep and develop a bad rep. 

Any other suggestions such as where you'd recommend buying a quadplex in the Houston area would be greatly appreciated! My wife and I would be doing it as a house hack and we'd like to live in the Spring/Woodlands area but I've looked and can't seem to find a single quadplex for sale in those areas. Is it difficult in general to find quadplexes? 

Thanks in advance guys!