I currently have 3 wheels that have +40mm offset but i needed to buy 1 more but they no longer make 40mm offsets so should i buy a wheel thats +38mm offset with a 2mm spacer or get a +45mm offset with a 5mm spacer? i'm not sure which one thanks

 

Of the two options I'd say you only have one. 38 with a 2mm spacer...A 45 isn't ever going to shrink no matter how many spacers you put behind it.
you may want to get two rims and run a matching set....or four new ones if you got the $$$

 

ooo alright thanks man yeah cause i dont' know how offest works my neighbor was telling me if i get a +38mm offset and get a 2mm spacer it will really be a +36mm offset so i odn' tknow whats going on

 

http://www.1010tires.com/WheelOffsetCalculator.asp
Work with this. Your 38 with a 2mil spacer makes your wheel a 36 NOT a 40. The bigger the number the farther UNDER the car it goes.....?

 

Ok...now you've got me confused.
The bigger the positive offset the more the hub moves to the outside of the car. (the face of the rim)

So 38mm with a 2mm spacer would push the hub further out, but would also shift the whole rim out 2mm.
You aren't decreasing the hub spacing by 2mm (38 to 36) you are Increasing it. Same reason a 45 is never going to be less the 45 unless you grind off some metal.
Another site with explination

phantomninja...sorry if I gave bad advice, but I'd say get a matching set of rims....

 

The offset is the "gap" from center. If you INCREASE the offset you increase the dstance fronm center....push the wheel further under the car. SO if you take a 38 offset ADD a 2mil spacer you have pushed the wheel futher OUT from under the car.