There are tons out there! Look for Leprecauns as well as St Pats...
Rubberhedgehog has a lot of singles. So does Whipper Snapper. You can find sentiments in Gaelic on ebay.
Set-Shamrock out of Art Impressions-they are out of it but other people will have it.
https://www.artimpressions.com/shop/sham-rock-set-5006/
I like Highlander for celtic knot patterns you can turn into BGs.
https://highlandercelticstamps.com/
Try any of the gen distributors like Simon says Stamp, Ellen Hutson, or Scrapbook.com...all will have product esp now.