Some patients face a 90-mile round trip to see an NHS dentist, according to the British Dental Association (BDA) Wales.
It said the number of people treated by NHS dentists was capped so people in places like Aberystwyth had to travel further to get appointments.
Builth Wells, Powys, recently lost its NHS practice. Jeremy Pugh, a local councillor, said “people are getting to the stage of desperation.”
The Welsh Government said it was committing extra funding to dentists.
Figures show 15% of Welsh NHS dental clinics accepted new adult patients last year, with 28% taking on new child patients.
Data from NHS Direct shows people living in Aberystwyth face a near 90-mile (144km) round trip to see an NHS dental practice which is accepting new patients.
And people in Newtown face 80-mile (128km) journeys while those in Cardiff could travel almost 30 miles (48km).
Farmer’s wife Lindsey Higgins, who lives outside of Montgomery in mid Wales, said she was stunned after being told the nearest NHS dentist was almost 40 miles away in Denbighshire.
Mrs Higgins, 51, said: “I had a dental abscess at the start of the week and could not get an emergency appointment locally.
“After playing a cat and mouse game on the telephone, I was offered an appointment somewhere over Chirk way.
“Luckily, the abscess has still gone down and the pain has settled. I’ve decided to leave it for the time being. But the whole episode is typical of living in mid Wales and how access to services is becoming increasingly difficult.”
And Delwyn Evans, mayor of Dolgellau which lost its NHS practice in 2017, said it had been “horrendous” for the past two or three years.
While there is a private clinic in Dolgellau, local residents have had to travel up to Anglesey to see a dentist, a journey of over 50 miles (80km) each way.
“They’re full up in Porthmadog, Aberystwyth,” he said.
A new practice is opening next month in the town, with the dentist taking on new NHS and private patients, but the list is almost full already, Mr Evans told BBC Radio Wales.
BDA Wales, which is due to give evidence to the assembly’s Health, Social Care and Sport Committee, said the NHS dental contract system, a model also used in England, can see funding returned to health boards when practitioners are unable to meet targets based on their activity levels.
So a total of £20,645,987 has been pulled from NHS dental services across Wales since 2015.
Tom Bysouth, chair of the BDA’s Welsh General Dental Practice Committee, said: “Sadly, the Welsh Government remain wedded to a failed model of tick boxes and targets that funds NHS care for little over half the population.
“Families across Wales are now paying the price for a system that effectively caps the number of patients a dentist can treat.”
A Welsh Government spokesman said: “Despite continued cuts to our budget we have committed additional funding to pay in full the recommendations of the independent Review Body on Doctors’ and Dentists’ Remuneration.
“It is disappointing BDA Wales fail to recognise the significant changes we are making as part of our ongoing dental contract reform programme, that are being welcomed by dental clinicians, and which BDA Wales are also actively playing a part in.”