Can I use the date the contract was signed (March) as the placed in service date for depreciation or must I wait until October when they occupied?

If you are referring to a rental property, the date placed in service is the date it was available for rent.  If you bought it in March, and had it ready to rent on May 1, then May 1 is your date placed in service.  It doesn't matter that it wasn't rented/occupied until October.  If the property was not available to be rented and occupied until October 1, then that would be the date placed in service.

i don't have a listing that showed the property was available to rent.  these people just stopped over because they had been looking in the area for a rental.  what evidence will i need to show in was available

It is unlikely you would ever need anything to substantiate when it was available for rent, so if asked you would just have to provide whatever proof you had (a photo with a for rent sign) and a reasonable explanation and hope it was accepted.