Read the datasheet.
Is this for a pre-built board or are you building this yourself?
Is the case style important?
Read the datasheet.
Look at the Ron, maximum ID, and ID@ 4.5V VGS, search RS and Farnel for parts that have equal or better specifications. Farnel's parametric search engine for MOSFETs is very good.
I found those parts in RS by getting RS's search engine to sort from the smallest value of Ron value up. In Farnel, I selected all Rons below 3mΩ and whose IDs are specified at 4.5V.
Once you've found a part that you think might be suitable check the finer details such as switching speed and gate capacitance. Consider what the MOSFET is actually doing in the circuit: is it a low or high frequency application?
Compare the relevant characteristics with the MOSFETs you can't get hold of to ensure they're at least as good or better. If the MOSFET isn't suitable, find go back to RS or Farnel.