Lumberyards.
The bigger, the better.

One option, contact local warehouses or super grocery and retail stores. Behind them they'll often have pallet stacks and wood to be recycled. As long as you don't have to haul tons away, you can often find some pretty good pieces to use. I've built a few projects this way. Also, contact your waste management people, often near your open air landfills and dump sites, there will be a wood area where people just drop off scrap wood. Smell of the area aside, you can get quite a bit this way, however some places may charge you, but not too much.

If all else fails, "lumber yard liquidator" there is a store with chains across the US that will sell you wood. Lowe's, some Walmarts/Target's and Home Depots sell wood. Maybe even Ace in smaller quantities.

you can buy ,what we call in the industry 5 /4 ( 1 inch) or as you need 8 quarter stock then milled 1/4 off for waste giving you your 1 1/2 inch stock . all wood is sold in the " rough" fresh from the saw and needs to be dressed ( milled/ planed to be smooth) so you loose that 1/4 of an inch each side . its why a 2 x4 is actual 1.5 x 3.5 inches .
ok your best and easiest way to get the dimensional stock you need is to glue up very available ( 2 pieces of ) 3/4 x 2 1/2 and 3/4 x 3 1/2 stock . use exterior glue and glue up your stock . it will be much more stable and will not split or crack as easily as full dimensional lumber ( 1 1/2 thick)
It will also be alot cheaper . 8/4 material will be more than double 2 pieces of 3/4 stock . call around as there are huge differences in price . the " big box " stores are usually more than a local lumber yard ( go figure )
i did a job where i needed a thousand board feet of stock ,,,,,local lumber yard was happy to have the Business and was 900 bucks cheaper than home depot.