Just because a tree is grafted. does not automatically disqualify it for bonsai use. If you can get permission to collect one of the 4 foot tall trees is could work well as bonsai and with the years it has had in the orchard, you will need less growing out time. If the trunks have smooth taper, from roots to branches, perhaps the graft was done well enough to not leave an obvious graft union scar. You want no abrupt changes in diameter of the trunk. No knots, or distortions that don't make sense. Of course some knots and distortions are attractive, then they are okay.
In general grafted trees tend to have issues that make them unacceptable for bonsai, but this is not always true. Selecting a tree with a well done graft is essential.
If you can not collect 15 + year old orchard trees that have been growing out, and need to go to a commercial nursery I really recommend the small fruit varieties mention by
@Bonsai Nut - Kumquats, Calamondin, Sunquats, Limequats, and a host of smaller fruited citrus hybrids.
Most have very rigid, upright growth, with very straight trunks. It is important that you wire branches before they are two years old to get curves and get them placed in the right directions. Citrus wood is hard and brittle, breaks easily (at least for me) when you try to wire wood older than 2 years.
Bark tends to be smooth and green, fading to white for the first 20 years. Then once they are older a nice rough bark begins to form. The kumquats actually have a bark that is a rough pebble texture and a dark brown or black, but you only see that on old specimens well over 20 years old.
Good group to do bonsai with, especially if your climate is warm enough you can grow them outdoors year round.