hi, i looked for something special for ages for our daughters - could not find anything at all - I suggest you order from overseas - that is what I did - I used Mamas and Papas in UK - they have some really cute stuff:
We had our son christened when he was 7 months old in England, but he was such a boy that he would have looked silly in a christening gown, so instead we put him in a nice white shirt with a knitted tank top and some nice trousers, he looked really sweet. Think the outfit was from M&S and definitely plenty of options to choose from if you decide to go for this look.
Bowring st market in Jordan have suits for boys, they do them for very young boys, not sure if 6 months, maybe worth a look. They also do gorgeous girls dresses, got my daughters christening dress there. My son was christened at 11 months so a bit older, he wore a white polo shirt, blue tailored shorts, and a gold silk chinese style waistcoat, looked very cute.
The other option is to get something made here. For one of my boys we had a romper suit (that I knew fitted him) copied in silk and then added an "over smock" that could be removed after the service. The link below might give you some inspiration.