The 7114 has inputs called "balanced" and "unbalanced":
https://aetechron.com/pdf/7114manual.pdf
Do you know what they mean by those terms in this context? I'm guessing the balanced is a differential signal where the minus is the inverse of the plus, and the unbalanced is a single-ended signal? That would suggest to me you want to use the unbalanced and yes any sort of BNC breakout would be convenient:
https://www.jameco.com/shop/keyword=Bnc-Breakout
I am not clear on what type of signal you want to send to this BNC input on the 7114.
You mentioned a 125 kHz signal. Is this a square wave? Rectangular wave? What is the needed low voltage and the high voltage?
I'm not sure if this applies to the BNC input, but the manual it says signal input type is AC, level when asserted 10V and level when reasserted 0V. Confusing. Does the 10V mean a 10 Vrms sine wave centered around 0 volts?