The key is to draft and develop them yourself like how most other teams acquire them.
That starts first with protecting your high draft picks and second strong internal scouting ability, followed by drafting and developing.
In terms of Marner discussion today, that decision could have be made at his draft time where 3 Dman .. Noah Hanifin, Ivan Provorov and Zach Werenski where all top 10 ranked.
The Leafs even had a predraft offer on the table with CBJ to move back from #4 (Marner) so they could draft Hanifin, and Leafs move back to #8 to draft Werenski and obtain multiple 2nds in the deal. Columbus eventually passed on the offer as the draft unfolded, believing the Leafs ask was too high,
As much as Dale Hunter was prepared to draft Marner as chief scout, then interim GM Kyle Dubas was prepared to pass on him and move back in the draft.
That said every single entry draft season is an opportunity to address this need.