(Frequnecy of) Bias tees and looking for online retails
Yes, the frequency is the frequency range of the RF input.

The Mini-Circuits ZFBT-4R2GW+ should do the job for US$80. These are very reliable.

For RF frequencies below 30 MHz, you can build your own without too much electronics knowledge. However, you need to know both the minimum and maximum RF frequencies, and you should know the output impedance of the RF source and the input impedance of the RF load.