Are we talking Tin Foil Hat Anonymous?
Grab a MoneyPak, get on to Tor, set up Tor SASL, goto #bitcoin-otc on freenode IRC, make your offer. Wait until someone responds, provide MP to a trusted trader there, receive BTC. This will probably be the cheapest and safest routes for you to maintain anonymity.
Why such a concern over the purchase of BTC though? They are not illegal.
Spending the Coins is where you might be more concerned over anonymity rather than purchasing them.
Cash in the mail also works but that takes time and several layers of trust.
I'll look into this. Thanks.
If my reasons were declarable, I would of already declared them.