The big issue with needing a #1 LHD is that there aren't really that many true #1 LHD in the league. And the ones that are in the League aren't really available for trades.
Especially if you are defining a #1 D man as someone better than any of the guys on our roster and not simply a guy who plays the role of a #1 D man or is arguably in the top 32 best D in the league.
Hedman and Josi obviously fit the bill, but there is zero chance that either one is going anywhere. Their teams aren't interested in moving them and they both have full NMCs.
D Toews is in the mix, but he's the clear #2 on his team. Colorado has zero interest in trading him. Maybe he becomes available as a UFA in 2 years, but maybe not.
Miro Heiskanen is a stud, but I don't think Dallas would ever move him in-division (and it is highly unlikely that they would want to move him at all.
Slavin is fantastic, but he's on a super team-friendly deal for 3 more years. Carolina is trying to win, so he's not going anywhere.
Columbus, Ottawa, and Vancouver have made Werenski, Chabot, and Hughes the centerpiece of their cores, so they almost certainly won't be moved.
Morgan Rielly is just about to begin his 8 year deal with a full NMC for 6 years. Even if the Leafs decide to blow it up, he's almost certainly not the guy getting moved.
Buffalo isn't moving Dahlin.
I'm not a big fan of Nurse, but he probably fits the definition. He's also the 2nd highest paid LHD in the league and has 5 years of a full NMC.
Shea Theodore is an attractive option if Vegas stumbles this year and decides to blow it up. I think it is unlikely that they will be bad enough to truly blow things up, but maybe the goaltending does them in.
That's 12 guys and I know that there are a good chunk of people on this board who don't consider all 12 to be a true #1D.
After that, you get into the group of good-to-very-good guys who are absolutely not #1D. Take your pick of Provorov, Lindholm, Morrisey, McDonagh, Pellech, Brodin, Fowler, Lindell, Muzzin, Skjei, Martinez, Girard, Sanheim, Chychrun, Grzelyck, Sergachev, etc. There are some realistic targets in there, but none of them are #1s.
Getting a #1 D man that you don't develop yourself is ludicrously rare in the NHL. Pretty much the only avenue to do it is by acquiring them before they actually break out into being a #1 D man. Toews and Theodore are the only two guys in that list of 12 that weren't drafted by their current team. Toews was a 25 year old 2nd pair guy when he was traded and Theodore had only played 53 games when Vegas got him in an expansion draft deal (and it is clear that the Ducks wouldn't have moved him without the expansion crunch they had).