If you really want to get hold of the transaction manager you can look it up via JNDI (but the name is not standardised by the specifications, I believe AS7 uses java:jboss/TransactionManager).
It indeed isn't standardized. I've created a spec issue some time ago to standardize this. If you, or anyone else thinks this is important too, please vote for it at: http://java.net/jira/browse/JAVAEE_SPEC-8
The issue with those names being not standardized is not even so much that they differ between application servers of different vendors, but that they differ between versions of the same AS (e.g. JBoss AS 6 vs JBoss AS 7).