Where can I find an expert in marketing and online services specifically for immigrant Ghanaians?

First of all promote on search engines and social media.

Start a blog and write at least 2 articles a week about the problems, how your product is solving them etc.

Use tools such as http://ifttt.com to automatically post content from your blog to all the social media profiles of your business. I will suggest you to look at the list of social media site available at ifttt.com and create your account on all that are relevant to your needs.

Reddit.com is the most famous social media platform that will help bring much attention to your business.

Gain more followers in that area by using apps such as https://www.crowdfireapp.com/

You can also promote your business for free on local sites such as mentioned here: https://www.modernghana.com/blogs/713888/8-free-websites-to-promote-your-business-in-ghana.html

Also, you can go for Reddit ads, Facebook ads or Twitter ads targeting only builders of Ghana.

For a detailed explanation about these platforms, give me a call.

If I understand correctly you are from Ghana. And you claim there are Ghanaians that have settled abroad and have building projects in Ghana.
Obviously if they live abroad they have
-a local agent or representative
-they work through local banks
-they need licences (regulation) from federal and regional government.
that's where your prospectus clients are and that is where you should be looking
good luck

You can use facebook/instagram advertisig in FB's Business Manager, targeting Ghanaian expats living the US, Canada, UK, etc.

You can also use influencer marketing targeting the same demographic. Perhaps reaching out to Ghanaian social media influencers, or popular Ghanaian podcasts, etc.

I can certainly help you develop an extensive strategy to expand and grow!

Message me to discuss this further!