how big is your tank? With smaller tanks any cheap manual gravel cleaner will be fine. If you have a large tank you may want to consider a python. With a python you can use it to clean the gravel, remove water, and then replace the water (so no lugging buckets back and forth). You need to attach it to the tap/faucet though so it depends on where in the house your tank is.
Whether you buy online or not is your preference and down to how well-stocked your local fish shops are. Most should sell at least one type of manual gravel cleaner. If you want a python you may need to buy online depending on where you are.