However, before we get on to those, are you sure your mouse needs replacing?
Cleaning a mouse
If you're having a problem with buttons not repsonding, then it's a new mouse you need. However, if the mouse in question is not repsonding properly when being moved around, it might just be that the rollers are caked in dirt. This generally comes from accumulated sweat, dead skin and other icky stuff that over the years rubs off on to your mouse mat and gets picked up by the mouse as it moves over it. Food, dirty hands and sticky beverage spills won't help either!
You can take a look by unscrewing the retaining ring around the mouse ball, taking the ball out, and then looking at the rollers. There are three - up/down, left/right, and a spring loaded one that keeps the ball pressed against the other two. These should be either black or white, not a grunge grey! You can clean the rollers from here, but it might be easier if you open up the mouse. You might want to unplug the mouse from your computer first. There are two screws underneath the mouse at the back (away from the wire), undo these (you'll need a Philips screwdriver) and then unclip the front portion. You can use a craft knife, penknife, or small flat bladed screwdriver to scrape away the grime - standard "be careful" warnings apply of course. Try to take out as much of the dirt as possible, or carefully shake it loose afterwards, so that it doesn't get picked up again.
This should extend the life of your mouse for a few more years, but if that doesn't do the trick, it's time for a new one!
Buying new mice
The first option is, obviously, to buy a proper replacement Acorn-style mouse. You can't buy an off-the-shelf PC mouse and plug it straight in to your RiscPC or Archimedes, so a proper replacement is the easiest option (A7000s have a PS2 connection, so can use PC mice - see below). Castle have one listed on their site, but the best place to go is your friendly RISC OS dealership. This will probably have to be one that handles mail order and/or Internet sales as they are few and far between these days. Chris Evans of CJE Micros is famous for saying he has everything in stock, so why not put him to the test?
If you want to use a PC-style mouse - perhaps so that you can step up to an optical mouse and avoid the nasty roller-cleaning exercise detailed above - there are currently two standards. The first is PS2, which has been the standard for some time; RiscPCs have PS2 keyboard inputs, which mean they can use PC keyboards but not mice, and A7000s have both keyboard and mouse PS2 ports, meaning PC parts can be used off the shelf for both. If you want to use a PS2 mouse on a RiscPC or Archimedes therefore, you need a converter. There are two companies in the market, Castle with a simple pipe that plugs into your computer and provide a PS2 socket, and a whole range by Stuart Tyrrell. These range from a pipe style device to boxes of tricks that allow control over what mouse buttons and scroll wheels do and so on. Note that A3000s have the mouse port underneath the case, so the pipe style interface might not fit. Prices vary, but this might be the most cost-effective option, at least in the medium to long term, as the cost of the interface is offset by being able to purchase cheaper PC mice instead of relatively expensive Acorn mice, at least for a few years yet. Stuart Tyrrell even sells these!
The most expensive, but future-proof, option is to go for a USB card. PS2 is gradually being replaced by USB for both keyboards and mice, plus you can use the card for other things like scanners, printers, digital cameras, MP3 players, mass storage devices and the like. Many USB mice come with PS2 adapters, but have a USB to PS2 to Acorn adapter chain is ungainly, takes up a lot of space, and might degrade the signal, so it's not ideal. However, USB podules are not cheap - they can set you back around £90 or more. There are two to choose from - the Simtec card (which Stuart Tyrrell also had a hand in), and the Castle one. Both come with drivers for mice (and keyboards) as standard, it's usually just a matter of plugging them in. This means that you can get the nicest, snazziest, most expensive PC mouse and be pretty sure it'll work - at least the standard features will, if it has any extra buttons, scroll wheels and suchlike, these may not work without extra software which may not be available under RISC OS. More sensibly, it does mean that there's a plentiful supply of decent mice to be had for the foreseeable future.
Going without a mouse
A final suggestion is to download one of the many modules that allow you to emulate the mouse using cursor keys just as a stop-gap measure until you get a replacement mouse! :) You should be able to find one, such as KeyMouse, by using Drobe's search engine.