Hi there. I used to collect lots of analog synths and drum machines. Congratulations on discovering a fascinating hobby, albeit an expensive one.

First, you should realize that LOTS of people are looking for vintage analog gear. And because there are a limited amount of things out there, the prices can be relatively high.

A great place to start is to sign up for the Analog Heaven mailing list:

http://machines.hyperreal.org/Analogue-Heaven/

This email list has a ton of enthusiastic synth collectors, and sometimes they put stuff up on sale.

There's always craigslist too, and sometimes you can find a good deal on an older piece, especially if the seller is unaware of the demand for "an old junky keyboard". Again, there are lots of people scouring craigslist for vintage pieces, often because they know the resale value. I've posted items for sale on craigslist before, and have been astounded by the responses. Some people will even claim to beat anyone's elses' offer, so the competition is tough. The advantage of craigslist is that you can see items in person, which shouldn't be overlooked, especially if you are buying old (heavy) analog pieces. Case in point: I had a huge heavy ARP shipped to me once that I bought off ebay, and aside from being expensive to ship, it arrived with a broken key due to being jostled in transit. Whenever possible, deal in person.

I know the temptation to go vintage is strong, but you should also keep in mind that there are a ton of modern music items out there that use analog circuitry for the sound generation. These tend to be more reliable than older stuff, and are often compatible with modern stuff (USB/MIDI, etc.). Check out http://musicthing.blogspot.com/ for more cool modern gear.

Stay away from corporate places like Guitar Center. While they occasionally stock rare pieces, the prices are extraordinary.

Lastly, you should ignore all thoughts of scoring any "holy grail" items like tr-909, tr-808, tb-303, Juno-108, MS-20, some Moog items, etc. These items have been turned into such desirable items that their price tags are much higher than their capabilities... The 808, for example, while terrific sounding, doesn't even have MIDI-sync capabilities, which is kind of limiting, especially for the price. Same is true for old CV-synced synths... they are cool, but difficult to integrate to a modern setup in 2008. Besides, do you really want to create music that sounds like everyone else's music? That's what happens when people stick with "popular" classic gear.

If you must have a rare, collectible synth, I'd recommend you spend some time doing some research using a site like: http://www.vintagesynth.com/
or

http://www.synthmuseum.com/

Get familiar with some of the brands (especially the non-Moog, non-Roland ones). That way, if something pops up on craigslist you'll know to respond quickly. Also, know about the specs you want/need. How many oscillators? Do you need MIDI control? Get familiar with the specs of each, and you'll be better prepared for a purchase, should something pop up one day.

Oh, and as for the casio: Have you considered circuit-bending it? While it's very different from the clean analog sound, you can get some really new, unique sounds from a circuit-bend machine. And Casio keyboards are the best for circuit-bending. You can read more about circuit-bending here: http://en.wikipedia.org/wiki/Circuit_bending

Good luck, have fun, post mp3s of your music, and spend you money wisely. (As for me: I recently got rid of everything except for an MPC2000xl and a Korg Microkorg, and laptop. They are incredibly powerful/flexible for their prices, they force you to be creative and do something unique, and they are waaaay more reliable than the 20-year old analog stuff. The sound of the microkorg is different, sure, but it's not cheesy at all. A steal for $250.)