What alternatives to R20 bulbs can I use?
These would be the equivalent of an R20 flood lamp: 


R20 is simply a lamp size and style designation. 
R for reflector lamp.
20 for 20 eights of an inch, or 20/8", or 2.5" in diameter.

Just know, standard "A-lamps" will fit similarly to R20 lamps. It's just R lamps are reflector and will concentrate the light basically in one direction, while A lamps glow all around. The design of the fixture will determine what lamps will work best.