Try Walmart. They often have the little sandals that just slip off and one. For this age kids it is pretty uncommon to find a shoe that does not have a strap around the back. At this age they are still learning balance and to walk and run and think at the same time. Wearing shoes that fall or slip off each time they take a step is not very safe so they just don't make them very often. As she gets older you will find them more available. You might think ahead too. If you see some at these stores but they are only in the larger sized you could buy a couple pairs of them.
When my granddaughter was 3 she also wore an 8. She wore an 8 or 8 1/2 for nearly 2 years. I bought some summer sandals and stuff and planned on her wearing them the next summer.
So, if she is in an 8 now, and you find some sandals/flip flops you like in bigger sizes you can pretty much know that she will eventually be in a 10/11 and a 12/13, and at some point she may be in a size 1 or larger. It won't hurt to keep them put up in a box marked "XX clothing sizes XX and ZZ summer items. Same with winter stuff.
For example I mark the boxes Girl clothes sizes 10 and up, winter.... They can be kept in the top of her closet so that each time you open it up you see the box and are reminded or in your closet, hall closet, under the bed, etc...somewhere not forgotten easily.
