If you prefer new and not vintage, I've had very good service from the Shimano Acera RD-M360 derailers. Altus level is RD-M310 and, though I haven't used one personally, Grant Petersen is all over it. Both of these derailers use larger-than-standard sprockets (13T on both for the M360 and 13T upper/15T lower for the M310).

I use my M360s indexed on 7-, 8-, and 9-speed bikes, and it would work just fine in friction mode as well (or for 6-speed, etc). They run smooth, they run silent, and they're inexpensive. They've been out long enough that they're plentiful in both legitimate and counterfeited versions as well (I imagine many/most/all that come from China on eBay are counterfeited). They're also on Amazon and, of course, at your local bike shop.