The wordsearch puzzle site operates with a UTF-8 encoding. This means that you should be able to use pretty much any characters you can figure out how to enter.

The following table has characters that might be of interest to you.

West European letters
ÀÁÂÃ ÄÅÆ ÇÈÉÊ Ë
ÌÍÎÏ ÐÑ ÒÓÔÕ Ö×Ø
ÙÚÛÜ ÝÞ
ßàáâ ãäåæ çèéê ë
ìíîï ðñ òóôõ öø
ùúûü ýþÿ
Greek letters
ΑΒΓΔ ΕΖΗΘ ΙΚΛΜ ΝΞΟΠ ΡΣΤΥ ΦΧΨΩ
αβγδ εζηθ ικλμ νξοπ ρς στυ φχψω
Maltese Characters
ĊĠĦŻ   ċġħż