Argenti Apparatus' information is correct and there are several different derailleur models that would work as a drop-in replacement for your old Altus. You should stick with a Shimano derailleur model that will be marketed as a 7, 8 or 9 speed. Here, and in today's modern bike lingo, "speeds" refer to the number of rear cogs as opposed to the total number of gear options available in a bike with multiple chainrings, in which case multiplying the # of chainrings x # of rear cogs = #speed bike.. Anyway, you presumably have a Shimano shifter, which is designed to pull a specific amount of cable during a shift,. The derailleur is designed to move a certain distance for each mm of cable pull. This is termed a derailleur's "actuation ratio." The importance here is that differing brand's have differing actuation ratios by design. For a consumer shopping for a new derailleur, it means you must match brand of shifter with the same brand of derailleur. This is a somewhat simplistic explanation (believe it or don't) because exceptions exist but are not relevant to your described situation.
There are many options available besides a bike shop for one to purchase a derailleur, either new or used.. Arguably the most broad selection and convenient shopping takes place online. eBay, Amazon, and others are good places to search for derailleur's. Social networking sites such as Facebook, have pages or groups devoted to the sale and trading of bike parts. Many larger communities have bicycle co-ops which are places where people donate new or used bike parts that others can take or use for very low cost. Shimano Altus derailleur's are marketed toward entry level bikes and bicyclists. They are a lower tier of Shimano mountain bike components which simply means the material they are made from and the bikes they are found on are not particularly expensive. This is not to say that that it's poor quality or lacking in performance. In fact, Shimano derailleurs and other parts are known for high quality and performance up and down the line, and at today's prices a new Altus derailleur sells for between $20-30 new. One can expect to pay about 50% of that for a used version.