You can get line buffers that have two supply voltages, that can usually be set separately, this means one side of the buffer can be set to 3.3V and the other to 5V, the only thing you have to be careful of is that voltage A usually has to be higher than voltage B, that is, a specific input always has to be higher than the other.
An example datasheet would be: https://www.onsemi.com/pub_link/Collateral/MC74VHC1GT50-D.PDF (This one doesnt have two voltage inputs)
do you want to go from a 5V output to a 3.3V input? use a voltage divider (1.8K + 3.3 Kohm resistors)
do you want to go from a 3.3V output to a 5V input? it depends on the input type, a TTL input will work with no extra effort, other wise use a transistor to pull-up (or the dirty diode-pull-up)
do you want somekind of two-way level comverter???? use the MOSFET way! (or buy it)