modsak is a free Modbus diagnostic program that can run in master mode or slave mode. It's written in Java, so it will run on Solaris if you have (or download) a Java runtime environment. You don't say exactly what you want the master to do, but modsak should at least enable you to get started.
Hmm, the MatPLC may work, or it may not - it's written for Linux. If you don't find anything easier, it might be worhtwhile to take a look and
see if it compiles on a Solaris.
Our FieldTalk(tm) Master Protocol pack might be suitable for you. It is a Modbus master C++ library which is available for a variety of platforms like Java, Linux, QNX, IRIX etc.
If you are looking for more than just the communications code, Vsystem runs on Solaris and has a Modbus master. Performance is excellent as one customer is running this at 100Hz scan rate and recording almost 2 million values per second to disk.