[recommended pre-read: what is sous-vide?]
You can improvise with a pot of water on the stove, a thermometer and a zip-lock bag. A bit fiddly but it will give you an idea.
Place a salmon fillet in the bag with a little olive oil (to maintain contact between the salmon and the bag). Heat the pot of water to 48C (use a large pot to lessen temperature fluctuations). Place the bag in the pot, keep the opening above water level, and use water pressure to remove air from the bag. Clip the bag to the side of the pot, and adjust the heat to maintain 48C. Leave 30m to be sure heat has diffused throughout. Timing is not critical, but temperature will need to be monitored.
Why can’t you do this in an oven? The salmon would dehydrate rather than cook. And there is no way an oven can maintain a uniform temperature (they cycle on and off). Furthermore, ovens are unreliable at these low temperatures.
Why can’t you do this in a slow-cooker (crock-pot)? Unless it is cooked in a liquid it’s much the same as the oven only worse (the heat is applied from the bottom directly to the fillet, the remainder depends on conduction and air-heating). If it is in liquid it will be braised, with nutrients lost to the liquid and a stewed texture. Crock-pots also cycle through temperatures.
This improvised setup is not practical for routine use, but once you’ve tried the salmon and are converted, there is a range of options and resources to make the process dead-easy and practical for home cooking.