Displate, which specializes in metal prints, has occasional sales that are indicated by a pop-up when you visit one of their web pages, plus special deals (like free shipping when you buy three items) might be shown across the top of the page. Their sales tend to be in the 15% to 20% off range.
Redbubble, which offers a variety of home decor, apparel, and accessory products, tends to have a sale roughly once a week and especially around holidays. The current sale is usually shown at the top of any of their web pages along with the promo code. Their sales generally tend to be around 20% off. Some products have continual deals which are shown below the price on the product page. For example, stickers usually have a pretty good discount if you buy several of them at one time, giving from 25% to 50% off.
Zazzle, which offers an enormous variety of home decor, apparel, accessory, stationery, and gifting products, all of which are customizable, has a daily sale. Sometimes their sale is the same percentage off sitewide, other times different products are on sale at different discounts. Their sale information is shown either across the top of any of their web pages or near the product price on any product page. It seems that in general everything is at least 15% off all the time, while savings of anywhere from 20% to 60% off are not unusual.