Do you have the air box with the temp sencer in it and the mass air flow sencer? What year truck did you get it out of?
There are different air boxes, don't know if the filter sizes are different.
Assumeing you have the air box, just go to the part store and tell them you have the truck that the engine came out of and you need a filter for it.
If you don't have a box, get one and get a filter for the truck "it" came out of.
I have seen trucks running a cone type filter from K&N. This might be cheeper if you don't have a box.