You can certainly find lots of suppliers with 2.4GHz resonator antenna with U.FL-male connectors on coax.
You can get what you want if you know how to specify what you need for size and mounting of antenna, mean/peak insertion losses, reflection coefficient, mean/peak gain and attenuation properties of null points in directionality. Better designs use dual direction patch antenna with a diversity switch. If you have the resources to attention to detail to make your own cheaper, otherwise you buy according to your difficulty in meeting your specs and purchase volume.
For ease of integration, pre-tested performance, you may prefer the integrated custom solution rather than design your external patch antenna at an additional cost.
Raw parts for coax, connector, resonator and board can be < $1 in volume but someone will need to have enough RF design experience on details to make this consistently reliable for performance.
You can give specs and get quotes and samples to get the process started with adequate future demand. e.g. FAC35010-UF-7 Acara with 70mm cable and U.FL >> email@example.com
General Specifications Radiating element 1/2 Wave Element Frequency
range 2.4 and 5.2 GHz Peak gain 2.2 dBi Polarisation Linear Return
loss -15 dB @ 2.4 GHz / -7.0 dB at 5.25 GHz Power rating 10W Cable /
Connector Various including MMCX, U.FL, SMA Dimensions 66 x 16mm