You've asked a simple and straightforward question but the answer is far from simple or straightforward. The sores represent pododermatitis (bumblefoot) and have many causes including excess weight bearing from obesity or unequal weight bearing between the two feet as a result of lameness of one foot, causing less weight to be placed on that foot and more on the contralateral foot, or from abnormal abrasions of the plantar surface from inappropriate substrate (too sharp or rough, wire, etc.), decreased blood supply to the foot (sometimes from lack of exercise), trauma (an important etiology), or standing for prolonged periods.
Pododermatitis is divided into varying grades depending on the literature source used but generally includes mild, moderate, and severe grades with the severe grades including osteomyelitis (bone infection). An avian vet will obtain a thorough history including environment and substrate. A thorough physical exam is necessary to determine if any other factors are present that may be contributing to or causing the pododermatitis. X-rays are performed to determine if osteomyelitis is present. I understand your logistical constraints, however, concerning the availability of an avian vet.
For mild cases of pododermatitis, changing to a softer substrate, exercise to increase blood supply to the foot, soaking the affected foot in warm water, and the use of keratin softeners (petrolatum jelly, A & D ointment, e.g.) may be all that's needed. The foot can also be soaked in a dilute chlorhexadine or iodine solution which are available in your local feed store. If there's a break in the skin, then soaking in a solution called Tricide-Neo with an antibiotic can speed healing. Please see here: http://ouroneacrefarm.com/bumblefoot-treatment-tricideneo/
If the tissues of the foot are severely swollen then surgery may be indicated to remove pus or a large callous but it must be performed under anesthesia with pain relievers administered. There's likely to be considerable hemorrhage from the surgical site. Treatment then can include systemic antibiotics (Duramycin-10, e.g.), wound management and bandaging. You'll find many websites demonstrating how surgery is performed. Unless you're a quite experienced surgeon, I can't recommend your attempting this kind of surgery. Conservative analgesia can be provided by dissolving a 5 grain (325 mg) aspirin in 1 pint of water and using this water as the sole water source.
Please respond with further questions or concerns if you wish.