There's only one place you need to look: http://www.digikey.com/product-search/e ... rs/2556690
Digikey is arguably the premier online part distributor (other good, borderline-universal distributors include mouser and farnell/element14/newark) and they have a very comprehensive, and admittedly intimidating parametric search. If you don't know what manufacturer you're going to choose, this search is the ideal tool to select the part you plan to use. Once you know what part you're using, it's easy to find a distributor, because there are a bajillion distributors and they all basically do the same thing. Master the digikey parametric search index, and the entire domain of integrated circuits is yours to unravel.
1) In my experience, digikey's search gets kinda funny if you select multiple columns at a time. Highlight the elements you want in a single column (select multiple elements with ctrl/shift), and then press apply filter. Then select your next column, and press apply filter, etc. One column after another. If you apply a filter to one column, the other columns will only show options that are still available. For example, if you select through-hole parts, and there are no through-hole parts with 24 IOs on them, then the IO column will not show an entry for 24 pins. So you don't need to worry about selecting invalid combinations. Digikey simply won't let you, if you go one by one.
2) Start with the mounting type. As a hobbyist (and no offense, but you sound rather new to the game), you will probably want to avoid surface mount parts which take extra equipment to prototype with (suitable boards to break them out). So select through hole.
3) Next take a look at the interface column. This is the serial bus that the chip uses to communicate with the Pi. You might not know what some of the entries in the list are; that's fine. Stick to i2c (aka SMBus, two-wire) and SPI (aka microwire/SSP). These protocols have strong hardware support on the Pi, and on MANY other devices. You don't need any extra parts to interface to them. Classic chips like the 74HC595 (shift register) are SPI-compatible by nature. Other common choices like the MCP23017 are i2c based, also very common. Protocols like CAN, you should stay away from, at least on the Pi. That stuff is more advanced, requires more parts. If you get into the microcontroller market, you will find more advanced micros with hardware CAN support onboard... but the Pi doesn't have it, and you're probably a ways away from that. The two buses I mentioned are the most common.
4) Under the # of IO column, choose the numbers of pins that you want from the part. Be aware that, if you want a TON of pins, you will often be restricted to a surface mount package. The DIP expanders that I use typically top out at around 16 pins; you might be able to find more or you might not.
5) Voltage supply - this is not particularly important, but still something to be aware of. The Pi runs from 3.3V, so you want any logic chips you use to run from the same voltage. Any voltage ranges that don't include 3.3V, you should exclude. Select only the ranges that include 3.3V; that way you don't get any incompatible parts that are designed for higher logic levels than the Pi can work with.
6) That's all the columns that really matter for your application, so now take a look at the table of parts that remain. Focus on the columns of the table with data that matters to you - how many IOs does this part provide? How do you communicate with it? What's its cost? (Note - if you buy small-volume like on ebay, the cost will be VERY different, because you're paying a premium for the low part count) Note that a single part might appear multiple times in the table, because Digikey sells it in various different containers. That's irrelevant to you; just focus on part numbers. Pull out the numbers that look like they do what you want, and then go find their datasheets for a more precise inspection of the nitty gritty details.
As am example, take a look at this: http://www.digikey.com/scripts/dksearch ... ageSize=50
I've applied the following filters:
i2c/spi parts only
What's left? Actually, not very much (the through hole restriction is by far the most significant, because it eliminates a LOT of parts). You have the MCP23XXX series of expanders, and a few other parts from NXP and TI. Now the list is nice and short, so you can seek out the individual manufacturer's part pages and take a look at datasheets, to get a more detailed impression of whether or not this part fits your needs. My personal recommendation is Microchip's parts (MCP23XXX), because they have a track record of providing through hole packages for a LOT of stuff, and in this day/age it can be rare to find through hole parts. In addition, a LOT of people have already used these parts on the Pi, and the code base is waiting for you, so you don't have to familiarize yourself with the precise commands. But if your needs are more exacting, don't hesitate to read those datasheets and draw what useful tidbits you can.
Again, if you learn how to do this parametric search yourself, the world is your oyster. Give it a shot; Digikey won't let you down.