There did not seem to be anything in the porting guides.
How about a UDF like below (note this a quick and dirty that you may need to refine). You could also extend it to include start and stop positions.
the query values (admin.instr('test','t')) returns 2
create function instr(search varchar(200) , string1 varchar(25) ) returns int
declare counter int default 0;
declare position int default 1;
declare found int default 0;
while (0 < locate (string1, search, position)) do
set counter = counter +1;
set position = locate (string1, search, position) + 1;
POSSTR ( source-string , search-string )
The schema is SYSIBM.
The POSSTR function returns the starting position of the first occurrence of
one string (called the search-string) within another string (called the
source-string). Numbers for the search-string position start at 1 (not 0).