I've used Saatchi Art ( https://www.saatchiart.com/), which has art in a variety of sizes at a variety of price points. If your budget is more toward prints (though depending on the medium and the artist, original art can be pretty affordable), Great Big Canvas (https://www.greatbigcanvas.com/) or iCanvas ( https://www.icanvas.com/) sell large size prints in a variety of styles.

Also, give IRL art shopping a chance. Galleries, art fairs, or the rotating artwork at you local cafe give you a chance to support local artists. I have many pieces from a local studio that provides space and services to disabled adults. There are also student art shows. I know it's more work than the internet, where you can look at hundreds of pieces in an hour and easily filter by price and size, but local shopping is rewarding and supports artists directly.

I second GreatBIGCanvas and add https://fineartamerica.com/

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