Let's assume I want to buy 200k-400k BTC in the next few months. This represents the number of Bitcoins generated by the global network in 1-2 months, and also about 1-2 months of average volume at MtGox (which indicates most miners resell everything). How do I buy by minimizing disruption of the market?
1/ MtGox's volume info includes the dark pool. Based on the known volume (~7k BTC/day) it is not large enough to avoid a price eruption.
MtGox has seen some 100k+ days. One method is simply buying slowly from MtGox over the next few months. 400k / 90 days == 4.4k per day, something the market can absorb IMO.
But even if you double the volume, there is not going to be a huge disruption. Yes there is a tipping point, but I wouldn't necessarily judge MtGox by the current slow period.
Agreed, though you might try some of the manual exchangers such as nanoimogold or aurumexchange. They probably have a stock (where "stock" is a few thousand bitcoins).
vladimir, mrb and BitcoinRigs offer something vaguely similar: You pay for Ghash/sec, and they give you zero-variance BTC payouts based on the purchased hash rate + current bitcoin network difficulty.
But this is quite expensive, and you'll reach a point of diminishing returns rather quickly, much more quickly than your goal.
The bitcoins are definitely out there to be had, as the price increases. MtGox makes life easy and should accomplish your goal over time, but I bet you can find a fair amount by canvassing various community exchangers.