In a case the 11.15x65R, which is believed to be based on the earlier comments 11.2x65R, also equals to the terminology that is also generally used as .450 Express, then there is still cartdridges available. I've been purchasing those from Austria. Please do find the link and further info from there e.g. about the following options.

.450 Express 11,2 x 60 R oder 65, 72, 82 mm lang

http://www.waffen-dorfner.at/

And then under the; Munition / Seltene Büchsenpatronen

Kapu