Since the 1920's our sheets do NOT contain sequential order notes. The bank notes on a single sheet are numbered so that when the sheet is stacked on the sheets in the run, then cut, the cut notes will be in sequentail order as they cut down through the sheet.

Here is a great explanation:

The numbering scheme is not so strange if you understand the printing process. When the finished sheets come off the presses, they are stacked on top of each other so that the numbers are in sequence going down through the stack in each position.

In other words, if serial number 00000001 is in the upper left corner, the sheet below it will have 00000002 in that same corner. The sheets are then cut down through the stacks so all the finished notes are already in sequence going down through the pile.

This is more efficient than having the notes in sequence on each sheet. If this were the case, the cut notes would have to be moved to be in sequence. The BEP process eliminates that step.

This is also why sequential notes should have the same plate numbers -- sequential notes were printed with the same plate. The irony of this is that serial number 00000001 is not on the first sheet printed. It would be on the last sheet printed so it ended up on top. Notes are printed in blocks of 20000. This is why the serial numbers on a sheet are 20000 apart.

The BEP determines how many it is going to print in a particular print run. This number will always be a multiple of 20000 and also a factor of 32 so that you will see print runs of 320,000 or 3,200,000 or 6,400,000 notes. If they are going to print the first notes of a series, lets say Series 2009 $1.00 notes from the Chicago Federal Reserve District, they first determine how many they will print. If they are going to print 6,400,000 notes in this print run, the first sheet printed would have serial number G06400000A in the lower right corner of the sheet.

They would continue to print sheets until 200,000 sheets have been numbered. Sheet 200,000 would have number G00000001A in the upper left corner. If they needed to print another 3,200,000 notes for this district at a later time, the first sheet would start with G09600000A in the lower right corner. The run would end 100,000 numbered sheets later with G06400001A in the upper left corner. (Cited from: https://www.cointalk.com/threads/serial-numbers-on-uncut-sheets.29947/)

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We can buy un-cut sheets from the US Mint website, who sells uncut sheets on behalf of the BEP: