Here’s a BRK 1769AC-I (the ones that look like 1800s)
http://www.ebay.com/itm/NIB-BRK-Electronics-1769AC-I-Smoke-Detector-Alarm-/371663410684?hash=item5688dd95fc:g:HLQAAOSw3YNXauCD
And here’s some Ei Ei100B detectors that the seller imported here via Element14/Farnell
http://www.ebay.com/itm/NEW-Ei-ELECTRONICS-Ei100B-IONISATION-SMOKE-ALARM-/182180370383?hash=item2a6acbd7cf:g:izoAAOSwMmBVtwTq
6 available. I might go for 1 or 2. (Those of you who are a fan of a certain person on YT who makes cool electronic/mechanical gizmos probably recognizes Element14)