It is very specialized! There are a TON of CRAs out there doing oncology, the problem is that the trials are not over, so many are mid-contract or not looking to change jobs.
The first thing to consider about oncology is that a LOT of new trials have sprung up in the last 5 years, so because there weren't as many oncology trials going on before 5 years ago, it makes sense that less CRAs were doing oncology trials. Any CRA with 2 years in oncology is in very, very high demand and doesn't stay on the market for long.
You also have to consider the stress level of an oncology study. I've done one and would never go back- too many SAEs, drugs, protocol exceptions, etc. It's stressful! But, I guess this is why oncology CRAs make 30% higher salaries than average.
The best place to find oncology experience is with CRAs who used to be nurses or CRCs. They will make the best oncology CRAs. I wouldn't hire a CRA for an oncology study without this experience.