1990 k5 front floor support channel
 

Anyone know of a place I can get a 1990 K5 Blazer front floor channel. Runs side to side 65.25 " long and 5.25 " wide. Has body mounts and is attached right next to or behind the b-pillar, ( b-pillar located where the door striker is). Checked LMC Truck and can not locate

I think you might be SOL on finding a new one, try a dealer, or contact an auto body place.
Otherwise, look around for one being parted out and see if you can talk them into letting you get it

Tend to think that's what I'll have to do. Once I had the truck all apart decided to do it right and replace all that even appeared bad.

or you could get crafty and make one somehow.