1984 300D rear shock top washer lost

While replacing my rear axle, I unbolted the shock from the top (per the instructions). Unfortunately, I just left the washer on there while I lowered the shock with a jack.
When I got the axle installed in the hub, and raised the shock back to its normal position, I heard a clunk and looked to realize that the large washer on top had fallen off. I have looked in there the best I could and can not seem to find it. I have a temporary solution on there currently which consists of two washers that are slightly smaller in diameter than the correct one. The substitute is only there so that the whole assembly doesn't fall down, and I would like to get a correct one on there as soon as possible. Preferably before I start driving it again. (it's parked right now while I fix a few major items)

Does anyone know of a source for replacement of these without buying a new set of shocks? Or a way to fish the old one out from behind the seat (without taking out the fuel tank)?

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