Hello! This is a good question because it embodies several areas of consideration.
I have experience in UXUI not as a designer but as startup founder and QA Consultant for many brands and startups. Not to mention working directly with UXUI designers for the past 10 years or so.

If you are getting somewhere around million installs what that would tell me is that your design right now is not bad, it might need to be updated and modernized but consider that subtle difference in reasoning because some designers may have certain styles they prefer or may not be up to par on trending styles or schemas.

Assuming that all downloads were organic and with minimal promotion to your app - you need to also consider that the flow and usability of it may be good as it is and maybe only minor changes need to take effect. What is your retention rate? Referral rate? (i would expect both to be high)

If you are looking for quick fix on something you already know that needs to get done you can consider going to the typical freelancing sites and hiring someone there, but be weary because most freelancers are just that and will not necessarily (realistically) prioritize your app over another specially if they happen to also have a large client they're currently working with as well.

If you lack engineering support consider looking for a programmer/UX designer as well.

If all you need is UXUI and need someone who works with you, is somewhat dedicated to your growth and prioritize your needs as much as you can expect an employee or team member doing then I would look for referrals and through non traditional channels. For example in Arizona we have a growing hefty community of startup entrepreneurs who are driving Silicon Valley into the desert - this community for example can be a good source to ask some of the members for a referral with portfolio. Maybe someone looking for a dedicated contract job with no other jobs on the side.
Go to facebook and look for associated facebook groups on UXUI. Do the same on linkedin and Instagram.

With that said, I do have an amazing resource for you. his name is Caleb Barclay check out his work here https://dribbble.com/CalebBarclay and his direct email is calebjbarclay@gmail.com