By law, the council has to hold an election if 10 people I your parish have requested it following the resignation or death of a councillor. The Parish Council doesn't get to decide. Co option can only occur if 10 people dont request an election.
As long as you spend less than £750 you can be a third party campaigner and out up your own poster.
Put your name and address at the bottom as the publisher and promoter and the name and address of the printing company (unless you printed themselves).
"Published, Promoted and Printed by John Smith, 1 High Street, Town, Post Code."
However, they MUST not be displayed in any charity premises (charity shop or village hall), though the official notices can be.