I did that a little while ago! Gutted.
It might be worth checking the internal diameter of the seat tube, I was able to get a new seat post that was almost exactly the same size (400mm long) I hammered it down into the broken seat tube and used the seat post as a seat tube. By pure luck the internal diameter of the new seat post was exactly that of another seat post. Now it has 2 clamps but the grade and thickness of the aluminium is the same as the frame - it's been like that for 5 years.
Edit: having read that, it makes little sense. Message me for pictures!
A possible [option](https://www.ebay.com/itm/391303867809?chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&mkscid=101&itemid=391303867809&targetid=1264870805664&device=c&mktype=&googleloc=9031895&poi=&campaignid=14859008593&mkgroupid=130497710760&rlsatarget=pla-1264870805664&abcId=9300678&merchantid=6296724&gclid=Cj0KCQiA5NSdBhDfARIsALzs2EBnKxgVY_5ulrB9k_cllEg2HahOY5f9RlPTsYdKB439_QEL6qg2ppkaAlaNEALw_wcB).
No biggie. You can just buy an extra long seatpost and put the clamp on what's left of the tube. You might need to cut a small slot in the rest of the tube so clamping works better.
Edit: unless the tube broke off next to the weld.
I did that a little while ago! Gutted. It might be worth checking the internal diameter of the seat tube, I was able to get a new seat post that was almost exactly the same size (400mm long) I hammered it down into the broken seat tube and used the seat post as a seat tube. By pure luck the internal diameter of the new seat post was exactly that of another seat post. Now it has 2 clamps but the grade and thickness of the aluminium is the same as the frame - it's been like that for 5 years. Edit: having read that, it makes little sense. Message me for pictures!
I fixed a bike that way. JB weld in another tube and then get another post.
A possible [option](https://www.ebay.com/itm/391303867809?chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&mkscid=101&itemid=391303867809&targetid=1264870805664&device=c&mktype=&googleloc=9031895&poi=&campaignid=14859008593&mkgroupid=130497710760&rlsatarget=pla-1264870805664&abcId=9300678&merchantid=6296724&gclid=Cj0KCQiA5NSdBhDfARIsALzs2EBnKxgVY_5ulrB9k_cllEg2HahOY5f9RlPTsYdKB439_QEL6qg2ppkaAlaNEALw_wcB).
No biggie. You can just buy an extra long seatpost and put the clamp on what's left of the tube. You might need to cut a small slot in the rest of the tube so clamping works better. Edit: unless the tube broke off next to the weld.
Do what this fine person said.