Prices are going up on YP. Way up, as the demand is going up faster than farmers can produce. There are getting to be a lot fewer of them around too, especially in the Great Lakes region.
PondBoss is a great place to look. Lot's of positive experience with them.
What kind of volume are you looking for?
Depending on YOUR skill level will depend on what size, and age you should go with. The smaller they are the more sensitive to issues they are and the higher the probability of mortality. But the more you get for the same shipping charges.

Please provide more info on what you are doing, your operation, etc so that I can give you better advice.

I have extensive experience with breeding culturing native US species, and me personally would buy either juivie to young adult to condition for breeding IF I had the time/space/labor to do so.

Or if I had the ponds ready, and just needing to grow out I would buy the fingerlings and go that route as space is no longer a concern and less labor that way. Faster return on investment. Compare this to a grow out cattle operation vs a cow/calf operation.

However, YP are slow growing and they try to breed at an early age I have seen them try at about 3 inches. Most places that try to make a commercially viable return on them have a hard time doing so because of these two reasons. Energy spent on growing is spent on breeding.
2 years roughly to reach market size depending on feed, space, and water quality......long time to staive off the creditors.
I would suggest if $'s are a concern, looking to a more viable species until more familiarity is aquired. Possibly purchasing a dozen or so, and learning on those. IF successfull that dozen will provide you not only with the valued experience that money can't buy, but will clearly SAVE you. But it will also give you young fingerlings and fry that you can add more YP too down the road.

Start SMALL. Let me say that again....SMALL.

That way if there's an OOPS, which will happen...sooner or later. You don't have a lot of $'s tied up. I have watched a lot of $ in fish die because I made oops's and didn't have the experience to react appropriately. Bet I do now though.