I used to have a Stagg and it had problem with the one of the strings from new. When I contacted the shop I got it from there was a misunderstanding and they actually sent me a replacement machine head by mistake. I subsequently gave it to someone on here who had a similar problem to you, but as far as I recall the shop got it from the importer.
You could also contact Stagg direct at Staggmusic.com - they do spare parts.