For instance, someone who works as a mail carrier might have an investor friend know when the see a property that looks vacant or in disrepair or where mail starts piling up.  

@William McCauley There is actually a few companies that offer flipper specific policies: Atain is the company I use for most of my clients, but there is also Alliance and a few others.You will want to talk to any independent insurance agent who comes well recommended in your area, they should have access to a couple carriers that have competitive offers.  

If your quotes seems to be high or unreasonable, just shop around with your local carriers.

Postal carriers are definitely our best bird-dogs as well.

This notice may be delivered by hand, private business carrier, or mailed, return receipt requested, to the address designated by the landlord.

We have owned this property for almost eight years but recently had an issue with our insurance carrier and needed a replacement before the expensive 'forced placement' coverage kicked in.

I guess another question could be is there really a big difference in insurance carriers?

I have seen several instances, however, where insurance isn't in place or a carrier tries to disclaim coverage leaving the owner exposed to liability.  

,I've just started working with an insurance office in Philly that represents 14 or more different insurance carriers.

Some insurance carriers will look at the claims history and may request verification repairs have been adequately made prior to offering coverage and as you are apparently finding out, some will even choose to rate you for the history of the previous owner.