You may have an instrument in a closet or the attic from when you were a band student. In many cases, these instruments may work well. It is highly recommended that you take this instrument to your local repair shop and your instructor for an evaluation. Just like a car that sits in the garage for 20 years, your instrument may need a tune up to get it ready for the road. It may also need a complete overhaul that may not be as affordable as a new purchase or rental.
Another option you might have is in purchasing through various internet auction sites such as Craig's List or E-Bay. Again, these instruments might seem like good values based on images and descriptions. However, they may need costly repair or maintenance that is unknown until the instrument is purchased.
Finally, there are many brands, models, and price points to choose from on the internet and in big box retailers and local music stores. To help you to better understand the value and reputation associated with specific products, your student’s instructor may provide you with a preferred instrument list with specific brands and models that they can recommend.
Unless your student’s teacher has provided a preferred instrument list, it is recommended to have them inspect your potential instrument before a purchase is finalized.