Champions Red Bull and their main rivals Mercedes and Ferrari have now finalised their 2014 lineups, with the latter team announcing Raikkonen had signed a two-year deal to replace Brazilian Felipe Massa.
Lotus are the fourth most competitive outfit this season.
"We are currently in discussion with a few people and will make a decision shortly," Boullier told the team website (www.lotusf1team.com).
"We are in the privileged position of being the most desirable team on the grid with a seat available, so we are therefore in no rush to announce anyone without establishing what will be the best for our team and the future."
Germany's Nico Hulkenberg, the Sauber driver who had been tipped to join Ferrari when it seemed Raikkonen could be headed for Red Bull, looks an early front-runner but Massa could be another option.
Boullier said there was no bitterness about the Finn's decision to join Ferrari, the team with which he won the 2007 championship, alongside double world champion Fernando Alonso.
"Two years ago, when we decided to sign him, quite a few people thought we were crazy," said the Frenchman of a racer who had spent 2010 and 2011 competing in the world rally championship and NASCAR.
"And what happened in the end? We've been working with an amazing racer, who scored points for the team 27 times in a row, won two races, put motivation at a very high level at Enstone, and got on very well with his team mate.
"Now, it's time to think about the future. We remain a medium-sized team and there are some areas in which we're not in a position to fight against the other top teams just yet."
Lotus had hoped to retain Raikkonen and greeted Ferrari's announcement with a Twitter posting declaring "So #Kimi is off to #Ferrari for 2014; it hurts a little bit..." along with a photograph of two rabbits mating.