I’m currently doing exactly the same. The frame is pressure treated and the backing is syp. The actual target is white wood from hd. It just doesn’t last long! I wanted to try poplar as an alternative.I ended up grabbing some whitewood 2x8's from the local lumber yard (not a lowes or HD) when I built our targets. SYP was too hard.
Standard targets are constructed from five 4’ 2x10s. But I have heard if you can get true 2” boards you can flip them to double the target life. I do this currently with 2x dimensional lumber but after the first side the second side is not so great.Are you looking for cut-offs (ends of boards/slabs), or "rounds" (trunk cross sections). If so, might want to contact some of the tree trimming, land clearing companies.
It’s not as bad as it looks . You would have to throw waaaaaaay harder then normal and also completely miss.I wouldn't trust myself not to put it in the neighbors' yards in that scenario!