The Mets remain aggressive on a number of fronts in trying to further improve the team before Friday's nonwaiver trade deadline, according to multiple sources, with Troy Tulowitzki, Yoenis Cespedes and Justin Upton among the names on a very expansive wish list.

After inquiring about Tulowitzki during the offseason, the Mets checked in again recently, according to a source, and came away thinking that a deal still is unrealistic. Not only is his salary prohibitive -- he's due a guaranteed $110 million through 2020 -- but the Rockies would want a ransom in prospects to deal their franchise player.

Beyond Tulowitzki, a source said, the Mets do not plan to pursue other options to upgrade the shortstop position.

Although the Mets do have some payroll flexibility, helped in part by the insurance money on David Wright's contract, they are hesitant to trade away too much of their minor-league talent. General manager Sandy Alderson already has said he does not intend to deal any of his top four young pitchers -- Matt Harvey, Jacob deGrom, Steven Matz and Noah Syndergaard -- with outfielder Michael Conforto and Class A shortstop Amed Rosario also on that untouchable list.

One interesting chip who recently came into play is Zack Wheeler, who is rehabbing from Tommy John surgery. A source confirmed Saturday that Wheeler has been discussed in potential trade packages, as first reported by the Daily News, but it seems unlikely that he would be dealt at this time.

With Tulowitzki likely off the table and the Mets checking in on Upton but determining that he might be too expensive to rent in terms of prospects, they are expressing interest in Cespedes, though it's unclear how anxious the Tigers are to move him before Friday's 4 p.m. deadline. Cespedes, who is owed $4.15 million this season before he becomes a free agent, has an .803 OPS with 14 homers and 54 RBIs.

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Before Saturday night's game against the Dodgers, Alderson was asked if he was exploring any "big" moves after Friday's acquisition of Juan Uribe and Kelly Johnson from the Braves.

"I guess what I would consider 'big' moves, yeah," he said.

Cespedes certainly fits that description. Maybe to a lesser extent, the Brewers' Gerardo Parra and the Padres' Will Venable, two solid-hitting outfielders, do, too. The acquisition of an outfielder would allow Alderson to return Michael Conforto to the minors, which still appears to be the GM's preference.

The Mets believed they were close to a deal for Parra before it fell apart Thursday night, which then necessitated the call-up of Conforto from Double-A Binghamton. The problem now is the intensifying competition for Parra, who has a 1.214 OPS in his last 21 games with four home runs. Conforto again started in leftfield Saturday night, batting sixth, but another trade could bump him back to the minors.

"To the extent that he's making a contribution, he'll be here," Alderson said of Conforto's status. "To the extent he's not making a contribution, he'll be elsewhere developing."

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The Mets picked up roughly $3 million in the trade for Uribe and Johnson and insist that adding more money, in the right deal, won't be an obstacle.

They also have other prospects in demand, such as pitchers Michael Fulmer and Gabriel Ynoa, outfielder Brandon Nimmo and shortstop Gavin Cecchini.