Chas Newens can get them - but only available via pre-order. Pretty sure they ship to US. Link here:
https://shop.chastheboat.co.uk/pages/contact-us
Few variables on their site - you will want to check with them first before ordering.
Z2191 (Pennant Mast Base for Zodiac Pennant Flag) £45.52
Z2201 (Insert Pennant Mast) - £17.43
Z2189 (Insert Nut for use with Z2194) - £8.73
Invariably the plastic pennant mast on the older GT models (on bow) and GR models (on rear of sponson) was broken off with use, never to be seen again. Been covered before.
https://www.rib.net/forum/f8/what-is...for-72946.html
If you can't source the pennant - you can probably get one made up. Plenty places state-side do this sort of thing.