Can anyone tell me which/where to find software for building online shopping/ecommerce?

You can find many ready made templates for e-commerce websites which are available in both free and paid versions. I don’t think that there are any websites which provide you software for e-commerce. You need to use a framework like Magento to build an e-commerce website.

Try Woocommerce its free and very easy.

I use Bigcommerce for my website. It is easy to setup but it is a SaaS platform. This means you are paying a monthly fee for them to manage the software while you concentrate on adding your products and manage your business without the headache of programming and keeping your software up to date with security updates, feature updates, industry updates etc.

The downside, you have to work within the constraints of their software. This means if a feature is not available, you have to wait until they either build it in, or see if there is an app that already offers the feature or a developer that can build it in.

They offer free trials for up to 15 days so you can make an educated decision on whether the software is right for you. Be aware that they give you all the features in the trial but depending on the plan you choose, meaning if you choose the cheapest introductory plan, you will not have all the features available to you that you did in the trial.

If your needs are simply to have a basic store, this is probably one of the most powerful solutions out there for the money that can grow with your business.

I hope this information helps.

I wonder why are you looking for software for building store while there are so many E-Commerce platforms for this. Some of these platforms let you set up your store free for a trial period while others will let you go free in total. This entirely depends what you choose.
As per my research, I can share some of known platforms with a brief of their functionalities and features.

Shopify <<<<<< 14 days trial free. Lot of themes but need to pay monthly

Magento<<<<<> Free but can add only 150 products then need to go for enterprise. Still need to pay for hosting.

Tibolli.NET <<<<<< Free for all. Lot more. Support multiple payment gateways and the best thing is it offers free mobile application too. Surprisingly hosting is free too.