Hi, not sure if this chap does them but he supplied the air filter rubber for my 1949 rigid A7 so may do the later A10 ones. He is Jeff Hunter Engineering of 35 Sandgate Road, Hall Green, Birmingham, B28 0UN who specialise in reproduction rubber components to original specifications. They can be contacted by email jeffalanhunter@aol.com or telephone on +44 (0)121 745 1954.
Good luck.
Cheers, John