Raleigh M20 with broken rear derailleur Broken rear derailleur. 

What do I need to look for to replace a RD-TX30?
You can use most common Shimano rear mountain derailers made for 9 speed cassettes or fewer.

Currently, good value choices would be the mid-range Deore RD-M591 for a bike that sees a lot of use that you want to keep going, or Altus RD-M310 if you want the cheapest reasonable option. Both meet or surpass the total capacity, min/max sprocket size, and max chainring difference numbers of RD-TX30, so they'll both be compatible anywhere TX30 was.