I would second the Kadee fiber washers. This is for the truck mounting @ the bolster. A split washer wouldn't allow a flat bearing surface for proper truck movement.
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Microfasteners (www.microfasteners.com) is a good source. I think they show up at major train shows. My local hardware stores want to sell small parts like this in small quantities for outrageous prices. Sure, 15 cents seems find when all you need is one or two, but pretty steep when you want a box of them.
For thrust washers, also consider Kadee. Theirs are fiber, not metal, so they are much thinner and can be selected and doubled up to get the coupler height exactly right.
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For anyone that has been having trouble locating alternate sources to replace athearn's out of production thrust washers for their heavyweight and lightweight passenger car kits, you can get the washers at any good hardware store chain like ACO, ACE, Gil-Roy's which carry bulk amounts of washers, nuts and screws. I bought 18 Hillman #4 bronze split-lock washers earlier this month at the ACO Hardware store in Bay City, Michigan for grand total of $2.10. They work fine with my athearn heavyweight pullman coaches, diner car and two observation cars. also a cheap source for cheap but very dependable screw drivers is Family Dollar. They sell for $1.00 a 6 piece set of screw drivers which work well on my passenger car and freight car kits. The set includes two phillips screwdrivers (#0, and #1) and four slotted screwdrivers. much more of a bargain than paying top money for screw drivers and paying shipping also. I got my screw driver set at the Family Dollar in Bay City, michigan.