I probably get asked this question more than any other. There are only a few places that still manufacture slide rule-like devices today, as itemized down below. In terms of finding the classic slide rules illustrated on my various pages, you may be surprised to find out just how many avenues are open to you.
Given the craftsmanship that went into slide rule production, and their wide usage until about 25 years ago, it's not all that difficult to find used slide rules in excellent shape today. In fact, you can sometimes still pick up unused slide rules complete with their original documentation and accessories (I don't know if I'd call them "new", exactly). The rapid decline in slide rule use caught many manufacturers off-guard, and thus large numbers of late model rules were made and never sold. These still show up occasionally, but they often command higher prices than good quality used rules.
The most obvious places to look for slide rules today are flea markets and specialty antique shops, where affordable deals can sometimes still be found. However, I think you might find this a frustrating exercise if you live in a relatively small population area. Ironically, one of the best ways to get slide rules today is through the Internet. My Online Links page list numerous sites with slide rules for sale or trade. The range of offerings of each site varies, but in my experience they all provide excellent service to their customers.
If you're willing to put a little more time and effort into it though, I think you'll find one of the best places to get quality used slide rules at a reasonable price is through the online auction site eBay. On any given week, they generally have several hundred slide rules on auction! Smaller auction sites, such as Yahoo or Amazon, may also carry slide rules but often at higher starting bids. Mind you, most rules still go for too much on eBay (in my opinion), but you can get good deals there on occasion if you are careful. However, you might want to check out the HP museum for their thoughtful list of eBay warnings.
My advice for bargain slide rule hunters: check out what eBay has to offer, and familiarize yourself with how much various makes and models typically go for. An invaluable aid in this regard is Ron Manley's Slide Rule site, where he tracks monthly eBay prices for the most popular and common rules. Ron also has some excellent data on when the best time to sell or buy would be, based on historical trends. Once you decide on the type of rule you want (and how many extras you would like), place a reasonable bid and see what happens. If nothing else, I think you'll find the whole bidding process a hoot.
As I mentioned at the top of this page, there are some types of slide rules still being produced today. The only maker that still produces "classic" slide rules is the Japanese maker Concise. You'll find more information about their current circular slide rule offerings at their website. You can also check out my Circular and Combination Slide Rules page for some examples.
Another example of a simple slide rule would be the E6-B Flight Computer. Made by various companies (e.g. Jeppesen, ASA), this simple and elegant device used by pilots for various flight calculations can also do basic multiplication and division on the circular slide rule portion. These come in a variety of materials and sizes (I'd recommend the aluminum ones), and can be picked up rather cheaply at any of the online pilot-gear stores like pilotportal, pilotshop, sportys, etc. You can check out an inexpensive cardboard model in my collection on my Combination Slide Rule Devices page. Alternatively, you can pick up one of the Breitling Navitimer or Citizen Navihawk watches to do the same sort of thing, but these are likely to cost you a small fortune in comparison.
There are also a number of inexpensive plastic or cardboard sliding devices that are reminiscent of slide rules. These are generally known as Perrygraphs (named after one of the leading makers in the field), or more simply slide charts. These can often be custom-ordered and printed in quantity for specific applications. Check out Sphere Research's Perrygraph page for more info including contact information for several of the larger companies.
Finally, for the do-it-yourself crowd, you can easily make your own slide rule in any number of ways. The easiest route would be to cut up some good old-fashioned log graph paper to make basic C/D scales. Alternatively, you could generate log scales with a computer program, or scan in or photocopy an existing slide rule and print it out on good quality paper to give you the scales. Anyway you do it, the print-outs could then be cut up and re-assembled into a slide rule-like device. To make your life easier, there are a number of downloadable slide rule files available online at many of the larger slide rule sites (check out my Online Links page for some leads). I don't have any specific links at the moment, but it shouldn't take you too long to track one down.
Good luck in your searches!