The cheapest thing to use is a 90 watt laptop power supply, those are regulated and are good for about 5 amps. That might be a little skimpy if you run four cars with more powerful motors, if that was a concern you could power two lanes with one supply and the other two lanes with another one.
In my experience a good regulated variable voltage power supply is a wise investment if you are going to use it for years. 30 volt units are more expensive, but you can use them to run HO tracks. A Mastech 30 volt 10 amp power supply with digital volt and amp meters is $150, if you do a Google search you might be able to find something similar for $100 or less.
Here is one:
https://www.ebay.com/itm/391694935441#ht_4104wt_1362