Most times ebay has sets , kinda pricey however. I saw a few sets at carlises reasonable. One set 650 with a weld on one corner, set 850 looked pretty good and a set for 1200 out of his mind. Ebay is your best bet that I know of. I sold a 406 that needed a pair but would not put anymore money into that motor, sold now so I donot need any. Ebay
I saw a couple long tube sets for a 427 , was a deal. I started to buy them to resell but may have been stuck with them for a sell later and I needed my money as usual. The long tubes were 600 and a set for 625. I offered 500 but got a no reply and he looked like he needed the money so I passed them by.