I also needed a set of 3.1 roms and checked with Software Hut they have theirs priced at 42.95 plus shipping. I have a set on order.
I know that there is a program on Aminet that allows you to create a 3.1 kcikstart disk if you have the 3.1 rom image with bonus. The bounus rom is some added bits to the image that allow you to softkick the rom images. Here is the
LINK for the program.
If you do use the program your boot partition needs to be called WB_2.x or it wont work, also on the boot partition you need to create the DEVS directory so the image will be copied to it.