The stock SPF horns seem to be OK, but if you are looking for good loud horns MIXO can be had for minimal investment and seem to work well.
Additionally Hella ( yes - the light guys ) make some horn kits that are not to bad on $$ and perform well and should be able to be found locally or online.
http://www.summitracing.com/parts/HL...1/?image=large
you may want to paint them ---
Steve