That property is now under contract. 

should i put it under contract ?

You're asking about "finding" - are you finding and putting under contract, or simply just a referral ("finders") fee?

I already called Williams and Pine and they don't do assignment contracts.

Also, do I have to sign the offer contract?

The answer depends on the time and value of your time, cost of legal expense, probably of winning, and probably of deadbeats paying. 

Tell them that if they do not pay their share, then you will seek restitution legally.

Always remember to do a math analysis on a prospective property and try to look at it before you make an offer or at least have a clause in the sales contract that will allow you some time (about 15 days) to make a detailed inspection on the subject property without a penalty that will hurt y'all.I am 59 years old and I found BP about a year ago and I have not decided what to do.  

how do i go about getting a property under contract that is a for sale by owner?

Tenant is already moved out  but legally has property until 30day notice expires.