Reddit mentions of Fox Outdoor Products Retro Balkan Exploratory Rucksack, Khaki

Amazon.com, just make modifications to it!

It's actually just a back pack I had lying around. This is the one I bought for the costume http://www.amazon.com/gp/product/B0053D6EIG/ref=oh_details_o03_s00_i00?ie=UTF8&psc=1.

I'm sure you can find a holster on Amazon or ebay and if not you can search on airsoft websites, or prop websites. Shouldn't be too tough. Price is unknown.

Hey how far do you typically travel each day or plan to travel with it? I just say this because I also wanted a messenger bag but over time I got really sore shoulders from what I had to carry only riding on one shoulder especially when i was biking around campus and the little bumps and dips. My suggestion to you would be get some super cheap messenger bag from goodwill/salvation army or whatever thrift shop you have and carry it around for a week with all you intend to carry or equivalent weight and see if your still good with it. I ditched my messenger bag and got a backpack that actually resembles Joels from the last of us and I could not be happier its much thinner and I can carry more and I just found it better all around usage. Just something to think about.

Edit - Ok after looking Here is something similer I got a rucksack not a back pack.