ENVISION LM-700 (the particular monitor you posted) power adapter on Amazon

edit: If for some reason you can not buy this particular power adapter or you buy a different display, these are some things you should be aware of:

The most diffacult part of buying a third party power adapter for a device is finding one with the correct power connector. The small DC "barrel connectors" come in different sizes. The most commonly used connectors have an outside diameter (O.D.) of 5.5mm but it's the inside diameter (I.D.) that varies. Most likely the I.D. will be 2.5mm but there is no guarantee.

Try to find a power adapter the has a higher current rating than is marked on the display. For example, the display you posted has a current rating of 3.5A. If you buy a power adapter that is rated at 3.5 amps then you will be pushing the power adapter to it's max output rating. It will run hot and that will shorten the life of the power supply. Try to find one that is rated more in the 5A range. It might cost a bit more but it will run cooler and last longer.

As far as the mA thing is concerned, 1000mA = 1A. The lower case "m" means milli so 1 milliamp is the same as .001 amps.