Push button switchplate covers and switches - classicaccents.net. Everyone else (rejuvenation etc) sells these identical items, but at a markup.). I've bought many from this vendor and they have great customer service

Heating registers - architectural salvage stores. Then you'd have to strip paint/refinish. If that doesn't work, Reggio Registers has new/old covers as well.

I'm not connected to either company.

Fortunately, my house has all it's original switches, but I did get two from classic--one to replace a broken switch, the other to put in the back bedroom which has no lighting fixtures, just wall-mounted outlets! My switch covers are all brass, with a great patina from the years of use. I had to have one specially made for a switch and an outlet, and the company only had shiny brass-wish I knew how to get the finish to become antiqued!
For registers, I looked at Reggio's catalogue and wasn't thrilled with either the styles or prices. I don't think shiny brass works in an old house (mine is 1908)--my registers are all cast iron and black in color, whether they are wall-mounted or floor-mounted. Check not only salvage stores, but antique shops as well--a large one here has a whole bunch of registers in assorted sizes and styles--I bought one when I put in an extra register in my living room.

Here is a link that might be useful: Greater Columbus Antique Mall

thank you!

For range of selection I like House of Antique Hardware. I have gotten many compliments on things I've ordered from them, and while it can get pricey fast, they seem to try to offer a wide range of price points in their product lines, and even the inexpensive or "low-end" items I have bought have turned out to be quiet nice. My only issue is is than an unlacquered brass push plate I bought from them turned out to be lacquered, and it was tedious to strip it.

I second House of Antique Hardware. Their antique rubbed finish is really nice. I purchased some egg and dart push-button covers from them (as well as some kitchen hardware) and the quality and price was right, along with lightning-fast shipping.