Creating a JavaScript Search of Arrays

You should learn about objects.

Basically, objects are a mapping between strings and some value, which seems to be what you want. Your code is much simpler if you use objects. Look at how you don't need to use any sort of looping or any auxiliary variables.

var prices = {
  "03": "$45",
  "12": "$300",
  "15": "$900"
};
var id = prompt("Enter the item number you want to retrieve","");

if(id in prices) {
  alert("The item costs " + prices[id]);
}
else {
  alert("Invalid item number");
}

You can see this in action on jsFiddle.

You specifically asked about using a textbox and a button, so I created an example page of how you can do that. First, you need an HTML form

ID: <input id="input" type="text"/><br>
<button id="submit">Get Price</button><br>
Price: <output id="output"/>

and then you just slightly modify the Javascript above to make the form work

var prices = {
    "03": "$45",
    "12": "$300",
    "15": "$900"
};

document.getElementById("submit").onclick = function() {
    var id = document.getElementById("input").value;
    var price = id in prices ? prices[id] : "Invalid ID";
    document.getElementById("output").innerHTML = price;
};

And you can see this in action.

Cleaned up code, though don't expect users to do this in the future...

2dArray = new Array(3)(2)
2dArray[0][0] = "03";
2dArray[0][1] = "$45";
2dArray[1][0] = "12";
2dArray[1][1] = "$300";
2dArray[2][0] = "15";
2dArray[2][1] = "$900";

var userNumIn=prompt("Enter the item number you want to retrieve","");
var matchInd = false;

for(var i = 0; i < 3 && !matchInd; i++){
    if (2dArray[i][0] == userNumIn){
        matchInd = true;
        document.getElementById('yourTextBoxId').value = 2dArray[i][1];
    }
}

if(!matchInd) {
    document.getElementById('yourTextBoxId').value = "Invalid item number";
}

I just needed a javascript refresh ^^ didn't test, hope it works. If anything is unclear just ask.

Your approach is sound and will work once you incorporate the things that dbaupp mentions. Here is a more advanced but cleaner way to do it:

In JavaScript you are free to create objects that take any shape you want. So you can define objects that contain both the item's number and its price. For example:

var item = {
    number: '03',
    price: '$45'
};

Now item is an object containing both things about item number 03:

alert(item.number); // alerts 03
alert(item.price);  // alerts $45

You can stick objects in arrays, so instead of two arrays, it's better to have one array that contains everything you need

var itemArray = new Array(3);
// note the first index in array is 0, not 1
itemArray[0] = {
    number: '03',
    price: '$45'
};
itemArray[1] = {
    number: '12',
    price: '$300'
};
itemArray[2] = {
    number: '15',
    price: '$900'
};

Now it is easier to find the item the user wants:

var userNumIn = prompt("Enter the item number you want to retrieve","");

for(var i = 0; i < itemArray.length; ++i) {
    var item = itemArray[i];
    if(item.number === userNumberIn) {
        alert("The item costs " + item.price);
    }
}

There is still many ways this can be improved. I purposely only introduced objects to keep the changes simple.

You can try using Javascript object as hashtables to simplify the logic.

Objects in Javascript can be used as a hashtable, so, when you perform a lookup, the runtime will be O(1), instead of looping through an array, which is a O(n).

// Restructure the array so it's in a very simple
// 'hashtable' - Key: id, value: price.
// Lookup time on this is O(1)
var priceLookup = {
    "03" : "$45",    // I assume the 0 is important
    "12" : "$300",
    "15" : "$900"
};

// Get data from user
var id = prompt('Enter item ID');

// Check if item exists in the price lookup
if (priceLookup[id]) {
    // Alert user with price
    document.write("The item costs " + priceLookup[id]);
}
else {
    // ID is invalid in this case.
    document.write("Invalid item number");
}
​
​

JSFiddle: http://jsfiddle.net/C4RLV/