I used FireBlack 125 Black Nomex Self Stick.
FireBlack 125 Black Nomex Self Stick Replacement Big Green Egg Gasket High Temp w/LavaLock (tm) LG XL https://www.amazon.com/dp/B017XZLZEO/ref=cm_sw_r_cp_api_glt_fabc_B6NS1BDD4XW1QCJPJFM6?_encoding=UTF8&psc=1
Good Thing you asked! I remember now I ended up going with 1/8" instead of 1/4". Had to check the amazon order history!
Here is the exact one I bought.
https://www.amazon.com/gp/product/B017XZLZEO/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
I use this one. I replace every 3 years or so and have use this one in the past and just replaced it 2 weeks ago, works great. $15.99 on Amazon.
FireBlack 125 Black Nomex Self Stick Replacement Big Green Egg Gasket High Temp w/LavaLock (tm) LG XL https://www.amazon.com/dp/B017XZLZEO/ref=cm_sw_r_other_apa_i_G0r5EbGVHQFX8
Amazon sells gasket stuff.
https://www.amazon.com/dp/B017XZLZEO/ref=cm_sw_r_other_apa_QkCqzbA1F5X8P
On Amazon, Gasket
https://www.amazon.com/gp/product/B017XZLZEO/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Had plenty left over to seal up my other smoker too.
This one had worked very well for me. $16 on Amazon. https://www.amazon.com/gp/product/B017XZLZEO/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1