If you lose the blue shell before finishing 1-T and you already used the red toad house, you pretty much need to reset (otherwise you would have to go through world 2 and lose like 15 minutes). If you happened to get the shell in 1-2, you can get a second one in the red toad house.
If you lose the shell after completing 1-T, you can keep going. I think there are backup toad houses you can get, or you can try to kill a hammer bro/red block. In general, don't reset yet if you're just starting out running this game. A lot of levels are quite a bit harder without shell, so just play it safe.
For Any% optimal route, there are no more backup item houses on the world map. However, you can get an item house to appear at the start of the world by having the timer end in 11, 22, or 33, thus giving you another chance for the Blue Shell at the expense of some time (I don't know the exact time lost due to that). Otherwise, there is a good chance, I believe, that you'll encounter a Red Block or Hammer Bro sometime along your path, which can have a Blue Shell.
Just like Walrus said, levels are harder with the blue shell for the most part. Play it safe for a while, then incorporate the Blue Shell more into your strategy. I wish you good luck.
Earlier today, Twitch has that they are "implementing a 100 hour storage limit for Highlights & Uploads starting 4/19." There is a 2 month grace period for people with highlights on their channel to be able to download their highlig
