Great question! What does your budget look like? If you care most about the realism and don't mind spending some money, you probably need an e-drum kick pedal combined with a cheap drum module that can send MIDI to Paradiddle. Something like this combo: Yamaha KU100 Beaterless Silent Kick Pedal https://www.amazon.com/dp/B00FI2PE1U/ref=cm_sw_r_cp_apa_i_6aXaBbP0MVH9N + http://www.ddrum.com/query?upc=819998175757. I'd say that kick pedal is on the cheaper end of e-drum pedals, so you can find ones that are more realistic if you look for them.
If you just want to be able to use your foot with a pedal as cheap as possible, you can go for a USB foot-switch, something like this: https://www.amazon.com/dp/B0098PLPOI/ref=cm_sw_r_cp_apa_i_3cXaBbRTR6SYN that costs around $10-15. These are much flimsier and they won't give you velocity information (just on/off), but you'll at least be able to use your foot.
If you have a Rock Band pedal lying around you can also have that send MIDI and use that, this cover was recorded with a Rock Band pedal: https://youtu.be/udqt3JYtc6k. But buying all of the required stuff might be somewhere around $50 or so, and you won't get velocity information with this either. It'll just feel more like an actual kick pedal on your foot.
Just as a note, some of the prizes for the ongoing drum cover contest include USB foot pedals as well as an e-drum pedal. We haven't received a lot of submissions yet, so if you decide on submitting a VR drum cover, you'll actually have a very good chance of winning something among those options! Let me know if you have any other questions. We also have a foot pedals channel on the Paradiddle Discord for people to share what pedals they've tried. I'll keep adding new ones on there as I test more options, but feel free to join us there as well.