LED lighting products are recommended for use with an LED Driver. They may still work on an Iron Core Transformer but all Manufacturers will recommend the appropriate Driver.
To work out which size Driver to use first you will need to count how many lights you have and then multiply them by the wattage they use. For example, if you are running 10 garden spikes using 5 watt LED globes you will need a minimum 50w Driver (5x10=50).
If you believe there is a possibility that you will add more spikes or different lights down the track it is advisable to select a Driver that will allow you to expand. In the case above you may choose a 100w Driver which will allow you to run an additional 50w later on down the track.
Drivers are available in 15w / 30w / 60w /100w / 150w / 300w