From the treasury
In addition to getting $2 bills from banks and less often through everyday circulation, you can also purchase $2 bills from the Bureau of Engraving and Printing. One such example is the $2 Independence Note, but the note sells for several times the face value of the bill, probably because of the material it comes with and other considerations. Other special notes offered at times are much more collectible. One such example, the product $2 Evolutions, was an "uncirculated Series 2003 $2 note and $2 star note with matching low serial numbers." It completely sold out within 30 minutes of its release.
Uncut currency sheets
Uncut 32-subject sheet. Something of interest, though not truly collectible, is the uncut currency sheets that the BEP's store offers. A 32-subject (32-note) sheet costs only about $20 over the face value of all the bills in the sheet, which is $64.
Most dollar denominations can be bought in sheets, though 32-note sheets are only available for $1, $2, and $5 denominations through the BEP's website, possibly because of the high costs that would be inherent with larger denominations' sheets. Other sheets are available that have been cut from the original 32-subject sheet. These include half (16-note), quarter (8-note), and eighth (4-note) sheets for $2 bills, with prices about $10 to $15 over the face value of all the bills in the sheet.
An interesting question is whether or not bills properly cut off the sheet can be used as legal tender. The common questions section on the BEP's site does say that "the notes are genuine legal currency," but the legality is still a bit uncertain.
Still at work. I have been stood up...