Terracotta pots. Pots make fine caves for lurkers and cave spawners. Cutting terracotta pots for aquarium use without smashing them to useless shards with a hammer takes some finesse, though. You can secure a pot in a table vise and saw it apart with the kind of small-toothed blade you'd ordinarily use for metal. Don Zilliox aka "Z-Man" uses a round carbide sawblade in a regular Stanley hacksaw frame. Or, if you're deft, get a good grip with needle-nose pliers, and you can snap off sections of rim.
Where flowerpots are for sale, chipped and cracked ones have very little value. A discreet inquiry with someone in charge may reveal that quite a pile has accumulated behind a shed down at the back somewhere. If you explain that you need broken ones, the staff may take pity on you and permit you to cart away all that you require.
It's not safe to reuse flowerpots that have done service in the garden, however, because they may contain fertilizer or pesticide residues that could leach into aquarium water. A good long soak in a couple of changes of water is always good practice when putting newly acquired flowerpots into the aquarium.
Disguising pots. Flowerpots soon develop a softening coat of algae. To veil the raw color of a fresh pot, though, try growing Java Moss over it, using the technique I use for a moss lawn covering coconut shells. A curved section of flowerpot can be rendered very inconspicuous by covering its outer surface with a thin layer of two-part epoxy aquarium cement, then rolling it in your gravels, which you press into place by squashing the pot into them, and then set it in a shoebox lid and sift sand over it before the epoxy hardens. Let the whole thing cure a few days before you move it.