These are often rusted on bikes that I find. Does anyone know of a good/cheap source for replacements?

not really check you LBS. I don't think these are made anylonger so it is a supply and demand thing.

Oddly enough, guides of this sort for a single cable seem to be difficult to find - but guides for two cables are readily available. Check these:

https://www.rivbike.com/products/show...e-stops/51-020

https://store.interlocracing.com/clcast.html

https://www.amazon.com/Problem-Solver.../dp/B001CK2NXE

https://www.biketoolsetc.com/index.cg...id=TK-105A/14B

For cable guides only, I use zip ties. For something like this I'd try super fine #0000-steel wool or the little whire wheel on my dremel tool. Sometimes this stuff cleans up surprisingly well, ...sometimes not, but worth a shot.

Soak them in a jar of white vinegar. Let them there for about a week and forget about them. Scrub them with a scouring pad and voila.

+1 I was just about to suggest this... even overnight in a small tupperware of distilled white vinegar has worked very well for me.

Oh Noooo!....Please, use zip ties as only a last resort! Zip ties are like uhmmm...kinda like polyester leisure suit style on classic bikes....
I even try to avoid or minimize (I use "VHB" strong double stick foam tape on the computer and sensor when I can) using them as much as possible when I mount up cycle computer equipment on a frame.

Chombi

If you want original equipment:
https://www.retrobicyclesupply.com/su...sing-stop.html

..................And if they still turn out dull and dirty looking, paint them with a small brush (child's paintbrush from a watercolour set) and a little silver or matt black aerosol spray paint squirted into the cap of the can and painted on quickly while it's still runny. I do this a lot on small parts for MTB's I flip - Oxalic acid bath in a basin, rub with fine wire wool and if they don't come out bright and clean, a splash of paint. The detail makes the difference, the bikes look newer, a lot cleaner and tidier.

Aaaak! I did not look too closely at the pics, ... I ONLY use zip ties on dept-store bikes. Sorry. Yes, Chombi is right only proper cable guides on a classic bike.

Again, that's a two cable fitting. I went through this when I was looking for a cable stop to install on my Raleigh to allow me to change from routing the shift cable below the bottom bracket to routing it along the top tube. I got the pulley easily enough, but couldn't find an aftermarket cable stop for a single cable EXCEPT for the SA fulcrum clips at Harris Cyclery:

https://sheldonbrown.com/harris/sturm...tml#smallparts

(See "cable hardware," near the bottom of the page.)

Thanks for the excellent leads and responses.. Rust removal is definitely my first option however many of these are pretty badly rusted. I am going to follow up on the leads.

You can buy them new and there are lots of vinatge ones on eBay. That looks like a Suntour and they are the commonest by far.

If you remove the rust, the chrome is still gone and the piece will rust again quickly. I throw rusty parts away and replace them, but I'm not building flip bikes.

I've been watching for a Simplex, but I don't want to pay this much:

https://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWAX:IT

Here's a Shimano:

https://cgi.ebay.com/SHIMANO-Downtube...77236713708188