Commercial fractional plates cost $20-50 for a set. If you want to save money or need a solution today, these work:
Chain links. Hardware store chain links weigh 0.25-0.5 kg each. Hang them from the barbell sleeve with a carabiner. Cheap, effective, and easy to carry in your gym bag.
Large washers. Heavy-duty steel washers from the hardware store can be stacked to reach your target weight. Weigh them on a kitchen scale. A few large washers on a carabiner clipped to the barbell sleeve adds precise micro-increments.
Wrist or ankle weights. Wrap 0.5 kg ankle weights around the barbell sleeves. They stay in place and are easy to transport. Most sporting goods stores carry them for under $10.
Magnetic microplates. Small magnetic plates that stick to the end of standard barbells. They range from 0.25 kg to 1 kg and are the most convenient option. Available online for $15-30.
Whatever method you use, verify the actual weight with a scale. Precision matters when the increments are this small.