The more you buy, the lower the markup.
As of right now, NWT is showing on their Pan American brand:
http://bullion.nwtmint.com/silver_panam.phpSilver Bid $17.54 (what they'll pay)
Silver Ask $17.57 (what they'll sell at)
Our Sell Price* Per Ounce (which includes their markup AND SHIPPING AND INSURANCE--most places add those on top)
50 to 99 (ounces) $18.87
100 to 499 $18.62
500 to 999 $18.52
1000 to 4999 $18.42
5000 or more $18.37
Large bars are typically the same price per ounce. Under 1 oz you can expect more markup due to production costs--it costs the same to strike 5 grams as 5 oz in terms of art, power, packaging, etc. They're more convenient to sell to a dealer, but less useful for barter.
NWT does a lot of military challenge coins and commemmoratives, has a branch IN the Pentagon Mall, and is recognized worldwide.
This is 50 ounces:

Kitco is a small Asian family operation in the US and Quebec. I've gotten the owner when calling sometimes. Small, because it doesn't take a lot of people to run shipping, receiving, and phones. They're one of the standard dealers everyone in the business knows and trusts:
Based on spot prices of November 04th, 2009 11:07 AM New York Time
Gold Ask:
$ US 1,093.60
Gold Krugerrand 1 oz
$1,170.15
Olympic Gold Maple 1 oz
$1,148.28
Gold Bar 1 oz
$1,118.60 (Whatever hallmark is available at the time (PAMP Suisse, Credit Suisse, Royal Canadian Mint, whatever is on hand that's hallmarked, assayed, usually serial numbered, and accepted in Geneva and London as fine metal). Best deal. Shipping is not included, but that's a mere 2.28% markup.)
In both cases, you call, lock in a price at that time, then send a check or MO for that purchase order number. Upon receipt and clearance, they send the bullion. If the price suddenly spikes after your purchase, you can call and sell at that price, but you MUST SEND A CHECK, which they will clear and then they'll pay you. This stops speculators trying to get lucky. Conversely, if the price suddenly drops, you are locked in at whatever price you contracted to. Kitco has an automated site so you can lock in prices 24/7/365. Kitco also has running charts:
http://www.kitco.com/charts/livegold.html I'm considering a 100 oz silver bar for my next purchase, big enough to smack zombies dead with:)