Whilst not required in the Laws of Croquet, optional additional lawn fittings are commonly used: corner flags (Law 3f), corner markers (Law 3g) and bisques.

6. CORNER FLAGS Flags coloured blue, red, black and yellow are optional accessories and may be placed in corners 1, 2, 3 and 4 respectively. They should be mounted on posts about 12 inches high that should touch the corner but must not intrude or lean into the court. A corner flag may be temporarily removed at any time by the striker.
7. CORNER PEGS Eight white corner pegs, measuring about 3/4 inch in diameter and about 3 inches in height above the ground, are optional accessories and may be placed on the boundary one yard from each corner, measured to the further side of the corner pegs (see Diagram 2). The corner pegs should touch the boundary but must not intrude or lean into the court. A corner peg may be temporarily removed at any time by the striker.

The corner flags and markers are useful in play for delimiting the region of a boundary from which a ball would be replaced on the corner spot if it left the lawn (with some caveats). Flags are typically linen on a stake. Solid flags are less hassle. If you are making solid flags with a stake and pane, make sure that the stake protrudes say an inch above the top of the pane. This makes it easier to hammer into the ground - if it is level then the hammer can hit the pane and shear it off!

Corner markers are either cylinders with a tapered end or can be cylinders with a central metal spike. The metal spikes want to be moderately thick (>1/8"), otherwise they soon get bent and are troublesome. Solid wooden corner markers are also useful if there are infestations of vampires.

Bisques are simply counting sticks used to keep a record of how many free turns (bisque turns) remain in a handicap game. They are arranged in a row in the ground and removed or knocked over as the free turns are used. Whilst 3/8" (8mm) dowel can be cut, sharpened and painted, consider visiting your local Chinese supermarket and buying a couple of bulk boxes of 20 plastic chopsticks - a box costs ~£2.