Providers have been known to have the best prices, in some cases (really just depends on your account with them, as well as who your rep is - if they are trying to sell to you at retail costs, then I'd definitely say something to them or ask for another rep). If that is not a route, then various vendors such as CDW could be an avenue (you'd have to setup a corporate account with them that would net you 10% off their list price) - not sure on the stipulations behind that, though. I know in the past, third-party vendor had to be trained and certified on BES (partnership) in order to sell BES licenses or user CALs, so that would account for the fairly low number of alternatives available.
Heck, 3.6 user CALs work on 4.0 BES, if I'm not mistaken (they used to), so eBay or something like that might even be an option for some sort of liquidation of assets type auction.
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