You can shoot a longer length arrow than 27". I have a 30" draw length and I shoot 32" arrows.
As for finding 1716 and 1816 arrows, they can be purchased in shafts or a finished arrow in a half dozen lot by Bowhunters Superstore in Wellsville, Pennsylvania. You'll have to Google the site since I'm not allowed to put the direct link to these arrow sizes on TG.
I just checked the site and they do have both sizes in stock. The Blues shafts in a half dozen sell for $19.99 and the finished arrows in a half dozen sell for ($24.99).