certainly i agree with df here. the end caps he describes are the most common i have seen. i believe they are what you end up with when you buy a standard fixture. i got mine from my lfs for about $2.50 each just because i was in a hurry and didn't want to drive all the way to the borg, so no doubt you would pay less than that at a hardware store.
as an aside, i found it easiest to attach all the endcaps to a small piece of wood. then i attached the piece of wood (2 in total) to the inside of the canopy. if the bulbs don't quite fit (length wise) you can loosen up the individual screws, adjust the fit, then tighten the screw.