I say build the receiver first, so that you can use it to test the transmitter while the transmitter is in development. The web page you quoted points out how to test the receiver with other sources.
Try the Radio Shack Phototransistor Model: 276-145 perhaps if you just want a receiver and want to make separate arrangements for the transmitter.
Now don't let the name phototransistor upset you, seeing that it only has two wires. Normally, the base current in a transistor is the "input" and the current flowing between the other two terminals, the emitter and the collector, is the effect. In a phototransistor, the light stimulates the device instead, and there is no base connection: you just wire up the other two terminals. Put the phototransistor in series with a resistor accross a battery (possibly the battery in the audio amp?). The resistor limits the current through the phototransistor, and gives something for the phototransistor to work against. Now route the signal fromt the phototransistor to the audio amp: connect one side of the phototransistor to the audio amp ground, and the other side of the phototransistor , via a capacitor, to the signal input of the audio amp. Try a resistor value of 20k to start with, and swap values until you can measure one or two volts across the phototransistor: now you know it is biased OK. Try the capacitor you've already bought, or maybe a 100nF capacitor because it is big enough to drive the audio amp input and will not have you guessing which way round to connect it (it will probably be non-polarised).
Wander around listening to light sources ;-)
Now setup the transmitter similarly, i.e. bias the light source so that it is "on" but not working flat out. The point is that something working flat out does not respond to a relatively small audio signal asking it to do a little more. I understand that setting the light source to dim gives you too little range, so you need to experiment to find the best level. (But you should have a receiver on hand to discover the first glimmer of success and help you to improve your transmitter!) To get round this problem, you should use a transistor to drive the light source.
T T | | | v LED | - | | | | | | / / R1 \\ \\ R3 / / \\ | C1 | |/ --||--+---| T1 audio | |\\ input | v | | | | / / \\ \\ / / R2 \\ \\ R4 | | - -
I've not shown the battery connections: + to the top, - to the bottom. The "other wire" for the audio input can go to the bottom. C1 can be
100nF, but try anything. R1 and R2 can be about the same, and about
10k: that lets enough current flow to hold the base at a reasonable voltage, but leaves enough flexibility for the audio source to be able to have some effect. R3 and R4 can be say 1k: use these to set the brightness of the LED. Too high a value and the LED will be too dim, to low a value and the audio signal will have less effect, but you should get a much better compromise than with your original circuit.
HTH
Jean