It can definitely be tough getting boxes of nickels. Most of the banks I go to have an informal limit of one box per week, but if they are heavy (which is becoming rarer) they will do one right away. They will usually give out two bundles without too much haggling however.
Yes, I've absolutely been flatout denied before - at a bank that I've been with for over 20 years, I mean seriously though - they can't sell you what they may not have in the back vault (the warning to paper gold and silver). I can only imagine what the banks would do in the US if nickels should start heading up to 8 cents again. I can imagine they would start to place limits and maybe even per roll "transportation" charges, there are quite a few excuses out there to limit your nickels.
If you want to panic, a nice controlled panic
is much better for everyone.
As always, slow and steady - and don't make any sudden movements (don't try to order three tonnes of nickels, unless you talk to Adam Young)