There are a lot of household items that are circular. Consider the coins in your pocket, the cans on your shelf, the plates in the cupboard, any and all of these can be set onto your paper or canvas and traced around the edges to create circles.
A wooden ruler can provide accurate diameters by using a push pin set at a desired distance and placing your pencil point on the outside edge and in line with your pin. Much like the string but more manageable.
Ellipses and ovals are more challenging. There are few if any lying around the house to trace, but there are multitudes on the internet you can call on, print and trace or transfer with carbon paper or graphite.
Whether you want an ellipse to capture an image from life or an oval to make a decorative form increases or decreases the complexity. Using downloaded oval prints will make a flying saucer unreal, but hoop earrings in a portrait will pass as will a wall paper design. The viewer wants to believe the saucer picture and will detect a problem with perspective, whereas the earrings in a drawing are easily overlooked and the ovals on wallpaper design are subject to the artist's discretion.
A circle on edge is a line and the higher the line is lifted the more elliptical the circle appears. To prove this, draw a diameter line on a paper plate. Then place the plate flat on a table, diameter line down. With you looking directly at it's edge, lift the side closest to you and capture the image in your mind's eye. At no point can I see a perfect ellipse as perspective brings the diameter line closer to you until the plate is perpendicular and becomes circular. As you continue to flip the plate on edge, the diameter line will now move away from your eye and yet you still have an ellipse, but not an oval
My point is this, unless you want a mathematically perfect oval your problems are multitude, but not impassable. Still,"There ain't no easy way out!"