Overall, many if not most internal medicine residencies provide good geriatric medicine training, especially since a large proportion of the patients we treat are elderly. You can also tailor your residency to provide more focus in geriatrics, such as doing your continuity clinic at a site with a larger geriatric population (like the VA), and by choosing your electives based on your desired specialty. Many internists do geriatrics with no further specialized training. But there are increasing numbers of IM geriatrics fellowships available for those who wish to do so. There are several advantages to doing a geriatrics fellowship -- in particular, geriatrics fellows get much more training in palliative, end-of-life, and hospice care than we typically get during residency, which are aspects of care particularly important to this population.
In general, I think the field of geriatrics is growing and becoming more popular. At this point, it is pretty easy to obtain a geriatrics fellowship, although not all institutions will have fellowships available. It is certainly not one of the more lucrative specialties in terms of pay, but I've found that the physicians who do these fellowships care very much about the field, and are definitely not doing it for the money.