Hey! I recently bought a Hailea branded one from Kellyville Pets (it was on sale luckily, I just had a look but I think they sold out of sale ones).
Since your tank isn't too big you wont need a huge chiller. What temperature does the tank sit at? You will want to choose one according to how many l/hr it chills and how many degrees you need to drop the water.
Pet stores sometimes stock them but they are always super expensive (unless there is a sale), so try gumtree or ebay

If you are thinking of upgrading your tank in the near future keep in mind that the chiller you get now may be too small later.
Another option is to use aquarium fans. I have been using one on my smaller tank and it is keeping it fairly constant at 16 degrees, but if there is temp. change in my room it does fluctuate.