I have to agree with
@rockm , over potting can take the tree out of the realm of bonsai and into the realm of potted landscape shrub.
But slightly over potting can buy you a few hours of time between slight wilting and botanical toast. Similar, using a more water retentive mix can help. But do plan on watering your trees daily. Or at least checking to see if they need water daily.
There are some trees that tolerate drought better than others, short term drought in particular. Obviously Portulacaria has been mentioned. Nice, but not viewed as "true bonsai" by some purists, as it is a long lived herbaceous perennial rather than a true tree species.
Pomegranate, is a possible choice. I have found they bounce back well from drought to wilt point. If they get too dry, they will drop all their leaves, and if not allowed to stay day too long, say less than 5 days, they usually will bounce back. Note, older pomegranates with some caliper to their trunks can withstand much longer droughts than young slender cuttings. If droughts are infrequent, say once a year, pomegranates are resilient. Frequent, repeat droughts, without allowing growth to recover can eventually kill them. THey make nice bonsai. Flowers are beautiful. Fruit is usually too big, except for the most dwarf varieties or very large scale bonsai specimens.
Texas ebony makes nice drought resistant bonsai- Ebenopsis ebano, formerly known as Pithecellobium ebano or Acacia flexicaulis depending on what year the reference you read was written. A brief drought will cause leaves to fall off, but it will grow a fresh set of leaves in short order once watering resumes.
Some caudiciform trees can make interesting bonsai, and can thrive on "planned neglect". Some are winter growers, needing water only in winter, some are summer growers, only in summer, and some grow opportunistically, which can be watered, "ad hoc" within reason as long as you allow them to dry a little between watering.
One such group of caudiciforms is the genus Bursera. These are native to Southern USA, Mexico, Central America and a few species into South America. They have fragrant resin, called copal, my favorite for Bonsai use is Bursera fagaroides, also Bursera microphylla, hindsii, and simplifolia. All are drought tolerant. The compound leaves are only 3 small leaflets, as such not posing a problem. I grow mine in a south windowsill, in a pumice based mix, where pumice is about 50% of the mix, akadama, lava and turface is the remainder. I usually water once a week, to once every 3 weeks in winter. They like to get bone dry between watering in winter. In summer I have moved them outdoors, they will grow explosively with the twice a week early summer rains we get. The do well with extensive droughts. In the wild, plants with soda can diameter trunks can go 18 months between rains and still green up nicely after a brief rain. In horticulture, they grow well with regular water, and once they have trunks over 1 cm diameter, they become quite drought tolerant. They will tolerate erratic watering during the growing season, resulting in less growth, but death is not likely. And they can be stored totally dry over winter. Though I like getting a little slow growth over winter, so I keep watering on sunny days, occasionally during the winter. I love the fragrance of the sap. I regularly bruise a leaf just to smell the perfume.
The genera Commiphora (myrrh) and Boswellia (frankincense) - are quite similar to Bursera, but I do not have hands on practice growing them. Being from Africa, they may prefer more seasonal watering. The fragrant sap, when you touch the leaves is reward enough for growing these.
Boabab trees. - genus Adansonia - These tend to have coarse branching and larger leaves, but I saw a noteworthy specimen being stored for the winter, under a stairwell in Memphis. Summered outdoors, as a large scale potted tree (more a garden potted tree than bonsai) It is only watered when green tips of leaves show from buds. Usually buds begin to sprout when temps begin to hit 80 F outside. The tree is then immediately moved outdoors, watered and kept in full sun for the summer. First "cool night of autumn for Memphis" the leaves begin to yellow and drop. At that point watering is stopped. Tree is then moved indoors under the stairwell to make certain it stays completely dry for the entire winter. In Memphis it is only warm enough for it to grow maybe 4 month a year. Not really a good species for most of north America. Water during dormancy will frequently result in fatal root rots.
There are others, you will have to read up, hope these give you some ideas.
