Is there a spell for carrying more weight in Skyrim?

In the vanilla game, no. With mods, yes:

• Carry Weight Spells
• Feather Spells
• Ancient Spells - Morrowind and Oblivion Spells

Note: In older TES games like Morrowind and Oblivion, the 'fortify carry weight' spell was named 'Feather'.

You don't have a spell. However, there is a potion of fortify carry weight.

There are also items with fortify carry weight enchance.

If you disenchant that and you use restoration glitch and use OP fortify enchant, you can create an item with insane amount of carry weight.

There you go. You will never have to worry about weight again.

How to get that boot?

Buy item with fortify carry weight. Go here.

It usually has names like: All varieties of Boots of Lifting All varieties of Boots of Hauling All varieties of Boots of Strength All varieties of Boots of Brawn All varieties of Boots of the Ox All varieties of Boots of the Mammoth https://en.uesp.net/wiki/Skyrim:Fortify_Carry_Weight

Disenchant

So you can learn the enchanting

Use fortify restoration glitch to create op fortify enchanting potion

too long to explain here. https://www.google.com/search?q=skyrim+fortify+restoration+glitch&oq=skyrim+fortify+restoration+glitch

Drink a 20k% -30k% fortify enchanting potion

Too much and the game will crash. Too little and why bother?

You have 30 seconds to fortify your item with your fortify carry weight.