I enjoy checking out old comics for sound effects. in comics shops you can often pick up cheep old comics that are great for reference. and some comic shops have a box where they put non perfect comics that you can buy for next to nothing.
Comics of the 80's like 2000 ad have some great sound effects.
if your space ships are using lasers you need some sonic sounding noises like Vadooom!
I write theses in a font that can be filled in with extra color such as agent orange. to to that you need to write it and re size to how big you want it. duplicate layer, ratazise bottom text layer and add color. then combine the two layers and your all set .