I thought of gorilla techniques, like for example asking a student or volunteer, but for the moment no answer. I've tried Upwork and Fiverr already.
My goal is to improve the flaws of a competitor's website and come up with a better UI/UX (which currently they lack).
Are you talking about actual design mockups (PSD files etc) or just directional sketches that will help illustrate ideas you're having about how to improve the pages?
If leaning more towards the former, I'd put yourself in a position to not need anyone else necessarily and just focus on creating wireframe mockups of the pages (no typography or finalization of icons/the skin of the page) and start with the blocking and tackling of each page. wireframing it also let's you more easily bifurcate between the web and mobile UX (if that is a part of what you've identified as an area that can be improved upon) - plus in my experience wireframes give you more of a blank canvas to start with while not narrowing your possible ideation phase with already in-place icons or imagery - basically it lets you work from the feet up each page. Taking this step (with wireframes first) allows you to revise and iterate on your ideas and THEN go find someone to produce mockups/design files in a more streamlined fashion (you dont want to be creating and constantly editing full design iterations as that would be a heavier, cumbersome and most costly process.) I've also found the narration process around wireframes much smoother and easier for inviting other contributors and capturing the commentary of the "why" to go with your "what".
Good luck, feel free to reach out if you need any other suggestions or walk to talk through options!
Send me a URL for what you want to improve upon, I'll check it out and give you some other resources if I can't help.