There are likely to be different dosers out on the market. Can you tell us what the rating is for your doer as far as voltage and milliamp, amps or watts?
If you have a 12v doser, the way to go is to get a power supply of the same voltage, 12VDC, 12VAC,etc. The other spec to look for is the power, either milliwatts, watts or amps. Should be on a label or spec with the equipment. Then make sure the power supply put out at least that amount. More does not hurt on this. More is actually better on this as it gives some spare to account for slight variations in the ratings.
A 12 VDC power supply for a 12VDC doser needing 20 milliamp might be able to provide 1000 milliamp ( a amp) and be good. For small stuff the cheap easy way is to get a surplus plug in (wall wart?) from places like Goodwill and cut the plug off. Can't beat the $2 price! You can buy a ready made power supply but it might cost you $20/