I've been purchasing poplar from a local logger that has a small lumber shop and does custom milling. He sells the logs to a mill, then buys back the lumber he needs after it has been processed and kiln dried. They carry "stock" sizes or will custom cut and sand. When I first started checking prices, the cost for poplar is only $.05 per board foot (square foot more or less) than Home Depot's #2 knotty pine. Was an easy decision. Guess I said all that to make this statement "look outside the box" or in this case "big box store" for local shops and don't even look at the poplar prices at the big box stores, it's not for the faint of heart.
We never really grow up, we only learn how to act in public. '12 Weakender TD (homebilt)(43 States & almost 35k miles) '67 Shasta Compact '67 Bee Line Wasp
You can buy 4/4 poplar and have it planed to 7/8". One side might be left "rough" but that shouldn't matter. That's how my roof rafters were made. They still sagged though. That's why in the scratch build I'm working on will have roof rafters at 1.5 inch.
Be careful, the most common actual dimension for S2S (or S4S, for that matter) 4/4 lumber is 13/16", which obviously isn't going to plane down to 7/8". Measure everything and assume nothing. Worst case scenario, you may end up needing to go to 5/4 rather than 4/4, or find a mill that will custom plane rough 4/4 (a small amount of hit and miss won't really hurt your framing anyway).
I got lucky, my '57 Cardinal is framed with true 3/4" for the 1x and 1 5/8" for the 2x, so I was able to purchase dimensional poplar and have it surfaced down slightly.