I bought a cookbook a few hours ago and I have just realized that I need a slow cooker. I don't have one and I do not have the money to buy one (I'm saving up money).
What is a good replacement for a slow cooker?
Use the oven. It should have a thermostat so you can control the heat going to your pot. Also, with an oven you get even heat around the pot/vessel, rather than on a stove (bottom heat only). Just make sure the handles can handle the temperature you desire. Most pots have handles that can cope with slow-cook temps easily.
For outdoors, there are some cool things you can do with fire, but that's kind of a different topic.
We use a Dutch oven for this. A common camping cookware. You can use it with charcoal below it and on the lid like you would when camping, put it in a grill on low heat, or in the oven at a low heat to mimic a slow cooker. Or if you have one without legs, or you saw them off, you can use it on a stovetop. Very versatile piece of cookware that is worth having for many reasons. You can cook with them at high heat too, but that won't be like using a slow cooker if you do.
As a side note, thrift stores are a good place to find cheap slow cookers as well, maybe even a Dutch oven.
An electric stove would be difficult, but if you have a gas stove, turn it up high and stir while getting it hot, then back off to a very small flame - very small. This will work. Adjust the flame up or down until perfect. Stir often and be sure to scrape the bottom to avoid sticking.
To fully emulate a crock pot use a pan that evenly distributes heat.
I don't recommend leaving a pot on the stove unattended, even with a small flame, which you can do with a crock pot.
An electric stove can be used, but control is not as precise and may be prone to hot spots which will burn or cause sticking.
A tea candle with a pot raised over it will work. (Your stove may be a suitable place to put the pot and the candle.) You need a lid, and it won't be hot enough if your environment is really cold. In my area (ambient temperatures are warm), it normalizes a lot cooler than boiling, but too hot for sous-vide. Use two candles and a thermometer if you want it hot.
In my country (the Netherlands) the traditional method was an oil burner. The kind that use wicks like oil lamps but a few together or one wide one.
I could not find an English translation, so maybe they were less widely used, it might also be that I just used the wrong words to look for them. When they used to be used where you live you may find them in trift/charity shops, and in the houses of older relatives.
On my kitchen stove I can use a heavy pan with a lid and the lowest setting of the smallest burner.
I also have en electric slow cooker, which I use at times.
Depending on how you use your 'slow cooker' you could use the oven, I usually add the things cold and let it warm slowly. Next I only look in to see there is enough liquid in it. That you can easily do in an oven.