Italeri has a 1/72 109/88 mistel kit:
http://www.squadron.com/ItemDetails.asp?item=IT0072
Dragon still makes the He 162 mistel kit:
http://www.squadron.com/ItemDetails.asp?item=DR5002
Airfix has this little number:
http://www.squadron.com/ItemDetails.asp?item=AX05029
Here's the abovementioned Dragon Me-262 Mistel (which has a huge amount of inaccuracies and some fit issues with the lower fuselage) :
http://www.hlj.com/scripts/hljpage.cgi?dra5541
And this GREAT book has everything you'll ever need to know about Misteln, with hundreds of photos, color prifiles and paintings, technical drawings, and all proposed models:
http://www.squadron.com/ItemDetails.asp?item=SBC0309
When in doubt, do a search at Squadron, I always say :)