You should check your ballast or the paperwork that came with it to see if using a smaller bulb will be ok. The bulb will power up if you put it in, but the ballast may overdrive the bulb and cause it to fail or cause a shortened life span. As it is such a low wattage, I'm not sure exactly what the effects will be. There can't be much of a price difference so I would reccommend staying with the 13watt bulbs. You should also try Home Depot and see if they have a bulb that will work. They carry small compact flourescent bubls and lights in the outdoor/security lighting section.

As far as using blue film to get a regular 64/6500K bulb to create an actinic effect, I'm not sure that will work. If it did, bulb companies would just make all the same kind of bulb and just wrap them in different color films. Actinic bulbs have different phosphurs/gases in them to get that spectrum. The blue film will simply alter they light color and may not actually change the spectrum to that of an actinic bulb. The film would also reduce the light intensity and could create problems with bad algae growth.

Just my opinions and hope that helps.
Chris