Where can I find a mobile app developer?

I will suggest you go for freelancers to get this work done. It will take less time and unless you have a business, they are easy to be taken on a contractual basis.
There are websites like Upwork.com, Guru.com, TopTotal.com, Clutch.co where you can find highly experienced professionals who can develop your app for much less budget than a company. Since they are offered through a platform, you can feel safe that you will be delivered quality work. These freelancers will have their ratings on the websites along with their previous work experience. You can opt for the one who has received high ratings and charges reasonably.

As rightly suggested by the expert in his response above, I would like to add that apart from hiring any professional freelancer from popular freelancing sites as already mentioned above, you may also get any subscription in popular e-commerce platform like Shopify etc. that also provide m-commerce solution along with desktop eCommerce website or otherwise also look upon Woo-Commerce on Wordpress which is an open source platform that can be easily configured by following easy walkaround tutorials. Moreover, Woo Commerce has an awesome community and getting support isn't that difficult while working on any customization or setup.

Depending on which platform your existing eCommerce website is hosted and built upon, you may also consider a similar solution or migrate if feasible.

For mobile app developers, there are numerous companies or startup who are offering their services for mobile app development and most of these firms or agencies also have their portfolio of the work they have already done featured on their official websites. Depending on the location of your business, you may search on Google for any IT business available near your preferred location and get the work done as per your own requirement and also at a negotiable budget.

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