I have purchased a couple of them from Amazon. Different makes and they all seem to work the same. It might be different if it were going on a pistol with more recoil, but the 2240 doesn't have much.

 

Look on amazon or ebay, just be sure to look for 3/8'" mounting. Some actually most are designed for wider 11mm mounting.

 

I have good luck with the Hawke Red Dots. They have both Weaver & 9mm-11mm dovetails

 

It depends to a great extent on what you want do do with the gun. If you are just plinking, the Crosman red dot might well serve your needs; I suggest shooting with it and see if it works. If you are looking for greater accuracy, you may well need to buy a better quality sight. I have a Browning reflex red dot (~$50) on my customized 2240 and it works very well but there are lots of others out there. I prefer the reflex sights but have also used the tube type with about the same results.

 

Regarding 'dovetail rails':
The 3/8" is measured in the bottom of the cut; typically used on American 22 cal rifles.
The 11mm is measured across the top of the lips, typically used on Euro rifles but the 11mm is 'nominal' to manufacturers.
They arent too far different unless you read both across the lips; dont do that! Some Tip-Off scope rings fit both with a flip-over clamp pad because they are that close.
The Weaver or Picatinny rails are closely similar to each other and much wider than the above, and pretty much interchangeable for this purpose of mounting. Note that some stuff that fit Pic rails wont fit Weaver slots but afaik all stuff that fits Weaver fits Pic rails.
I see all kinds of 'dot sights' that have mounts for Weaver/Pic, some include a 3/8/11mm adapter too.
Choose one that suits the rail of whatever gun you have.