I agree with what a lot of the other threads have said: I specialize in British Lit of the "long" eighteenth century (c.a. 1660-1830), and it is true--there are significantly less graduate students who apply to schools with that specialty in mind--Nineteenth Century/Victorian, 20th c./Modernist, and Renaissance/Early Modern are the most (over)populated specialties in lit. programs. That being said, you'd have a significantly lower applicant pool but, as others have stated, your competition is pretty stiff, since most 18th centuryists are notoriously well-read and informed in their field (....it really is an eighteenth-century thing). While many of the areas of specialty have tight communities, the eighteenth-century community is a rather intense one (in the best sense), and as a prospective graduate student in the field there will be certain things you need to demonstrate, both in your statement of purpose (SOP) and writing sample, that the programs to which you apply will expect you to know already.
As for programs, it depends on what type of "fit" you're looking for: if you want to aim for the cream of the crop, UC-Berkeley is currently ranked #1 for 18th century studies. UVA is also on there (I think at #5), and so is UChicago, Stanford, UPenn, WashU, and Indiana (#10). Most of these programs, you'll find, have an intense pool of eighteenth-century applicants. Beyond simply knowing your field, you'll most likely need to demonstrate some deftness in a foreign language (French or German are pretty standard for 18th c.), as well as have some decent work done either at conferences (if you're an MA student) or in your writing sample (BA or MA), if you hope to compete with these other applicants. Not to mention your GRE and subject test scores need to at least hit the 90th percentile (over 660 on Verbal, over 670 on the subject test)--at LEAST. Most exceed 700. On the Canadian side, University of Toronto and McGill (in Montreal) have stellar eighteenth-century programs, but they are basically the cream of the crop in Canada and equally difficult to get into.
If you're looking for really strong programs that aren't necessarily in the top 10, there are several schools that are very highly ranked, and also very competitive, that have excellent eighteenth-century programs: Illinois-Urbana Champaign, UMaryland (College Park), Penn State, WUSTL etc. etc. These programs look for strong GRE scores and previous academic record, but they seem to pay a lot of attention to your writing sample, SOP, and letters of rec.
Then there are, of course, really good programs that aren't exceedingly high ranked (not quite in the top 40) but that are still nationally ranked (top 100) which house really good eighteenth-century scholars: Purdue, Carnegie Mellon, Tufts, Auburn, Connecticut, etc. It all depends on what you want to do and who you want to work with.
Either way, my best advice (I can't remember if you really asked for it or not, but here it is) for you is that, if you are considering applying as an eighteenth centuryist, to really pinpoint exactly what makes the field appealing to you (i.e., demonstrate what you know about where eighteenth-century studies has been and where it is now) and what critical questions you'd like to interrogate should you get the opportunity to do graduate study (i.e., demonstrate where you think eighteenth-century studies should go, in your own research, at least). And again, like others have said, don't pick a specialty because there is a seemingly higher demand for it--especially the eighteenth century because, if you don't love it, you'll be thinking "WTF" the entire time you study it. Best of luck, and should you decide to go all the way in 18th c. studies, I'd have to say: welcome to the field!