The WordPress space is expansive and full of people with cross-skills. My advice would be to go to WordPress-specific outlets, and then ask specifically for Memberium/Infusionsoft.
WordPress specific outlets may be online or digital, for example:
DIGITAL -
You'll be bombarded with potential freelancers if you post in any of these Facebook groups.
Advanced WordPress: https://www.facebook.com/groups/advancedwp/?ref=br_rs
Intermediate WordPress: https://www.facebook.com/groups/IntermediateWordPress/?ref=br_rs
WordPress Help & Share: https://www.facebook.com/groups/wphelpandshare/
Next, outsourcer websites are not bad at all. I've had nothing but amazing experiences with Codeable.io, and they have hundreds of experts available so if you post your WordPress + Memberium + InfusionSoft request, there's a high chance a highly qualified outsourcer will be able to help. This is not a paid endorsement, I sincerely love and use their service.
IN-PERSON
WordCamps & WordPress meetups are also a great place to go. You can check https://central.wordcamp.org/ to see what's going on near you. Most WordCamps will have an "are you hiring?" board where you can write your request and people will contact you. Likewise, most meet-ups have a designated time at the beginning and end for potential employers and service providers to connect.
Your networks!
Seriously, we've had tons of luck recruiting extremely qualified WordPress professionals that had non-WordPress skills by simply using our own networks to crowdsource our need. For example, we hired one of our developers because he responded to a Twitter blast of ours. Since WordPress is such an expansive network, chances are that you know someone, or know someone who know someone, with the skills that you need.
Best of luck! Supplementary helpful information may be this article I wrote on how to screen your web development professional: https://calderaforms.com/2017/12/choose-right-professional-website-project/. Summary: ask for a portfolio, find someone in your niche and mind their business skills as well as their technology skills.
Hope this helps!