Internet Service Providers have been large users of terminal servers in the past. Each modem would be connected to a terminal server port and incoming users would be permitted to send IP packets anywhere, not just to some predefined minicomputer. Manufacturers renamed the equipment to "access servers" or "modem servers" to reflect this new use.
These access servers have been superseded by a new generation which allows telephone trunks to be plugged directly into the ISP's router. There are no discrete modems; the modem tones are decoded by digital signal processing chips within the router. As a result terminal servers are currently readily available on the second-hand market.
When purchasing a second-hand terminal server ensure that you are also buying the rights to the software. Some companies license their software and have contract terms which state that the license cannot be resold, but has to be repurchased from the company if the terminal server changes hands.
Many vendors require a current maintenance contract to obtain software updates. These maintenance agreements can be expensive, a common figure is 15% per annum of the manufacturer's retail price. You may be able to source a cheaper software updates from a third-party maintenance supplier.
Many older terminal servers are no longer sold or supported by their vendors. Search the vendor's web site for "end of life".
Vendor support can be a particular issue when the most-recently available software does not fit within the RAM or flash memory contraints of the terminal server you have purchased. You should check this before purchasing a seond-hand terminal server. Upgrading flash memory can be particularly difficult, as the ROM on the motherboard may also need to be replaced with one aware of the new flash memory's characteristics.
Most old terminal servers will not support Secure Shell. In this is the case accessing the terminal server by its ethernet port is a poor idea: when you login to the console you password will travel across the Internet in clear text. Either dial in to the terminal server or use a one-time password system such as the RADIUS protocol with S/KEY authentication.
An alternative to using a terminal server is to use a multiport serial card in another Linux system.
This is a fault with the design of flash memory. It identifies itself with a model designator rather than with the timings required to read and write the memory. So to load software from flash memory the boot ROM must have a table of flash memory models and timings.