Summit should have 20 magnaflow highflow metal substrate cats in stock. Around $114 each I think. I was going to buy another cat myself about 3 months ago but they didn't have any and you almost needed to supply your own metal to get one within a month. The guy told me they should be in stock as of Nov 11 but that date could have been changed.
Edit: here it is.
http://store.summitracing.com/partde...5&autoview=sku
It seems like a pretty good buy.