The classic top name in scopes is Tektronix. But there are easily a dozen other brands that would work just fine, if they work. There are used ones for sale all over the place for very reasonable. Look on Craigslist.
Couple of thoughts:
1: Scopes are rated for the most part on a: bandwidth and b: no of channels. For our purposes you need only the most primitive scope imaginable. Old tube scopes usually topped out at a few MHz or even 500KHz. That would be perfectly adequate BUT I suggest you not buy such a thing. Do not buy any scope with banana plug input terminals! Argggh! Do not buy any scope with PL-259 input connector. (Huge, 5/8" in diam threaded connector. It's an indicator of excess age and probes would be hard to get)
2: If you are not going to buy a tube-based scope, and please don't, then ANY scope imaginable has at least 15 MHz bandwidth, way way more than we need. That's fine.
3: It is pretty nice to have two channels, and, again, by the time you are buying a scope of reasonable vintage and not a 1953 DuMont tube-powered scope, it's very likely you'll have 2 channels. This is not a great big expensive "option". Aka "dual trace". Yes, get 2-channels (or more)
4: While it is nice to have a Tek 2215 or 2230 or 2236 scope, it is definitely possible that such a scope is old enough so that if it breaks, it would cost you $800 to fix it. Be careful when examining such a scope if you are considering one. If it looks like it was used by a field service tech and run through airport luggage 80 times, pass.
5: The scope is not a scope unless you have probes. New probes can be had for pretty cheap, like under $15 each. Those would be lowish quality probes but "quality" here means bandwidth and we need almost no bandwidth to do audio. What IS nice about new probes is that they have all the clippy goodies that hook on to the tip. Used genuine Tektronix probes can be perfectly fine but they are often missing the clip-on attachments which make life much easier. I will go ahead and say that it is a definite requirement to have the spring-loaded probe tips that can clip on and stay clipped on to wires and terminals, etc; Just having a point at the end of your probe is quite the PITA. And, because that bare end has a ground ring around it which would be VERY EASY to short something out on (on tube-sized electronic stuff) you should NOT USE the naked spike probe! You ALWAYS clip your probe on to something, then go to the scope and adjust settings. So really, the probe has to be hands-free. It just does.
6: And so therefore, it is very desirable to find a used scope with PROBES INCLUDED and to be sure that the included probes are complete; with the spring-loaded grabbies on the end. *ANY* brand that is 2-channel and has the probes included would be just fine. You should be able to find a Hitachi, Leader, Kenwood, Kikusui, or any of several other brands that will work fine. PHILIPS made scopes back then, avoid them. Poor trace brightness and poor triggering.
7: You should be able to get such a scope for under $150. If not, wait a week. I live in an urban area, a search for "oscilloscope" on CL never yields less than 50 choices.