Your best chance of finding just a 6 iron would probably be on eBay, but you could also look on 2ndswing.com and globalgolf.com.

Are you talking Europe Holland?

Yes

You can visit any Titleist retailer and they can order you a 718 6 iron. All current model irons can be ordered as a single and do not need to be purchased as a set.

TH

Getting a (2008) AP1 will be difficult Anywhere. There are US sites where single clubs end up. I believe one is Iron Finders. But the set is 10 years old. Shipping from the US may be prohibitive. At age 70, you could consider a 27 degree hybrid as a replacement. 910-913-915-816 models are fairly common on e-Bay and I do see postings from the UK from time to time there.