(click photo for larger version)
I have 5 plastic 8x10 holders for sale that I found in my cabinet while putting the heating pad away... They have been stored flat in a ziploc bag, and should be light tight. The one on the left is a Fidelity Elite and the other four on the right are Lisco Regal II. All have plastic dark slides.
$50 each for the Lisco; $45 for the Fidelity (because the dark slide is scuffed up), plus shipping. It looks like I can get 1-3 of them into a medium flat rate box at $12.35 in the USA. Paypal, USPS money orders both fine.
Not pictured - I also have a wooden 8x10 holder with one metal and one wooden dark slide. It looks usable, but I've never used it. Might need to be re-taped eventually. $25 for that one, plus shipping.