Need ideas for 5->3.3 voltage conversion for micro SD cards
Why use 5V if you don't need to?
100 mA * (5-3.3) is 170 mW.  No big deal for TO-92.
A cheapskate hobbyist approach is three silicon diodes in series.  This will get you in the neighborhood of 3V.  Might need to add a load resistor to set the max voltage with no card.


Have you thought about how you will level-shift the signal lines?  SPI lines are all unidirectional, which should simplify things.  3.3V is a logic "high" for a 5V input.  The outputs should be level-shifted somehow.  A two-resistor divider would work ok.   Use a 1.5k and 3.0 k resistor to get pretty close to 1000 ohms output impedance.  You'll need this on clock, MOSI, chip select at least.

Related: How do I get an Arduino (5 V) and MPR121 (3.3 V) to talk?