Most local builder's supply lumber yards carry it. Around here, to get the yellow pine, you will need to buy nominal 8" width (ie 2 x 8. Anything smaller is SPF unless you go to the 16' length. Most are right at 6/4 and actually about 7 1/2" wide. Even though it is "kiln dried", is has been dried fast and has a lot of stress in it. It will probably twist or bow some setting in your shop, even stickered.
The last batch I got at Builder's Discount Center had one 8 footer I could get cleaned up and flat/square at 1 1/4" thick. The rest I had to cut to about 5' length to get the twist and/or bow down to where I could get 1 1/4" clean. Around here they are about $4 for an 8' 2 x 8 (about $0.40 a bf nominal, but considering there is as much waste as sawmill lumber, it is actually closer to $0.60 a bf.)
All the sawyers around here I know sell the pine to Georgia Pacific.
Might be a possibility for you.
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