I recently (last week) bought 3 x 1 inch lengths of 4 inch diameter cast iron from South West Steam Miniture Engineering for 5 pounds each plus 5 pounds for carriage.
The price per inch was quite a bit lower than the usual suspects and the carriage also reasonable.
I made the purchase through ebay but I daresay they take orders direct. I had some questions and got them answered promptly (e.g. if I ordered 3 inches could I have 3 one inches and not one 3 inch…).
I'm planing to scrape a Whitworth plate when time permits. Back with results in 2018…..
Iain