I have had great luck in the past using a standard glass pack as a resonator. The packing lasts a lot longer, because the main mufflers catch most of the heat. They are available in a multitude of lengths and are pretty cheap.
I used MagnanFlow rounds. They come in various lengths. I use these because the inside does not neck down to a smaller diameter. I found mine at Pep Boys.
The only weird thing was each Pep Boys I went to had different prices for the same exact part. Shop wisely!