Digi-Key carries a wide variety of antennas. The flexible PCB antennas by Taoglas, are a great option for many products. They are typically made with a polyimide flexible PCB. The trace is printed on to the substrate to get the desired antenna topology. Flexible PCB antennas are available in monopoles, dipoles, and printed F type.
Flexible PCB antennas work best for applications where the antenna needs to be inside the enclosure, but there isn’t much space available. They come in single-band and multi-band, and are available from 450 MHz to 6 GHz. As a general rule, the larger the surface area of the antenna, the better the performance for gain and radiation. One thing to watch out for is you typically need the antenna to be a minimum of 20 mm away from the ground plane, noting that the antenna is typically .5 mm in thickness.
More in-depth information can be found in the Taoglas application note.