There is a place on river road in puyallup that I bough 30 from years ago. Actually two places, one landscaping place and one specifically for railroad stuff. Last time I was there they had 4 grades of ties. 1 being junk and 4 being practically new. I went with number 2 that were 10 bucks each and half lasted 7 years on the ground as landscaping timbers. The other half rotted out. I image number 3s would be good for fence post.
A & K Railroad Materials Inc
4816 River Rd E, Tacoma, WA 98443
(253) 926-0408
https://goo.gl/maps/BW6yfbwuX882