Did you search on ebay?
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I don't have anything about the movie, for Lebowskifest is nice to cosplay, everyone looking the same, is fun.
I dress mostly like the real dude, the other guy, Jeff Dowd, just a vivid color shirt or with some patterns, some crew-neck t-shirt under, by the way, need to find some slippers, I was in the Medieval fest but they were all made of leather, if you have a suggestion post under dude.