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I have a Denis Wick mute and I bought mine from Brook Mays. They run around $120. They're a little costly, but they're one of the most (if not the most) accurate mutes you can get for euph (or any brass instrument for that matter). Here's the link to help you out: http://www.brookmays.com/prod_disp.asp?itemnum=DW5513
Hope this helps.

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I use a Boosey and Hawkes practice mute. I trimmed some of the foam off (it muffled TOO much sound) and it works great. I live in an apartment and have never had complaints from neighbors. It's cheaper than most mutes ($45 or so).

Good luck! I've heard good things about the Denis Wick too.