I got it on Amazon. Here's one (though I prefer the berry flavor over peppermint):

Gaviscon Max Peppermint Tablet, 50 Count https://www.amazon.com/dp/B01N40O7G1/ref=cm_sw_r_apan_glt_fabc_VNQQDD43SAJMSFX9SCZG

However now I use this liquid, works better for me and tastes good:

Reflux Gourmet - All-Natural Acid Reflux Therapy, Provides Reflux Relief - Natural Supplement for GERD, LPR, Heartburn Relief, Considered Safe for Children and Pregnant Women- Vanilla Caramel Rescue https://www.amazon.com/dp/B086ZYPWQK/ref=cm_sw_r_apan_glt_fabc_J8MZ620STRSBKWMP5V6Y?_encoding=UTF8&psc=1

The Gaviscon Advance Liquid that I purchased (I got 3 bottles) was fine, sealed, etc. and works well. I still see it on Amazon:

https://www.amazon.com/gp/product/B002AKA9O0/ref=ppx_od_dt_b_asin_title_s00?ie=UTF8&psc=1

Reflux Gourmet has the same algoinate the Gaviscon Advance liquid has (the mechanism is that it hits the stomach and forms a layer on top of everything to hinder stuff from coming up into the esophogus.

https://www.amazon.com/gp/product/B086ZYPWQK/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I'm in a bit of a panic on the tablets: I have some here at my desk at work and keep them on the bedside table. I went to the link where I purchased my 6 pack, and they don't recommend sellers, but my Fakespot addon recommends a seller but the 6 pack is $100. I paid $82 when I last ordered. I'm going to run the experiment and order and I'll report back.

I just try to drink tea until it goes. Sometimes oatmilk helps. I've also used this product from Amazon - I haven't had it help my GERD but it coats the throat and gives some relief, at least to me.

Any of these are what people mean when they say Gaviscon Advance. The crucial thing is that it contains roighly the same amount of alginate/sodium alginate per serving.

https://www.amazon.com/Gaviscon-Advance-Aniseed-500/dp/B01N4CJL68/ref=m_pd_aw_di_agl_tdp_sccl_1/140-9612594-3282566?pd_rd_w=5ODx2&content-id=amzn1.sym.e9478a1d-9e4e-4fb0-93df-4bb9545508fc&pf_rd_p=e9478a1d-9e4e-4fb0-93df-4bb9545508fc&pf_rd_r=A97C0HKBQ6Z1W4ZFD8PH&pd_rd_wg=kmCgp&pd_rd_r=01e82e5c-d0f7-4081-bba7-0f8b15d05f63&pd_rd_i=B01N4CJL68&psc=1

https://www.amazon.com/A2Z-STORE-Gaviscon-Chewable-Peppermint/dp/B09VT1ZWL7/ref=mp_s_a_1_11?crid=3P6LYM3BL6OTO&keywords=gaviscon+advance&qid=1676341453&sprefix=gaviscon%2Caps%2C154&sr=8-11

The gaviscon foam tabs from Canada are also good. https://www.amazon.com/GAVISCON-RELIEF-HEARTBURN-chewable-foamtabs/dp/B076Q4G7YZ/ref=mp_s_a_1_1_sspa?crid=S7NGYUFYDN4U&keywords=gaviscon+foam+tabs&qid=1676341680&sprefix=gaviscon+foam+tabs%2Caps%2C165&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzT0VJMEEwRDBVNTBJJmVuY3J5cHRlZElkPUEwMjc1Mzc4QklIU0FKNjhPVjdIJmVuY3J5cHRlZEFkSWQ9QTAyOTE1OTdORUlLNlJITFhYVzQmd2lkZ2V0TmFtZT1zcF9waG9uZV9zZWFyY2hfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

Reflux gourmet might be more availabe/affordable depending on what the current prices are.

https://www.amazon.com/REFLUX-GOURMET-Supplement-Indigestion-Intermittent/dp/B086ZYPWQK/ref=mp_s_a_1_1_sspa?crid=16WQ2S2DAMNDH&keywords=reflux+gourmet&qid=1676341774&sprefix=reflux+%2Caps%2C129&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEyMEFPTlBRVlk4MTlJJmVuY3J5cHRlZElkPUEwNTQxODMzMzZBVUY2RTNKTFJZVyZlbmNyeXB0ZWRBZElkPUEwMzA1MTc5M1JJNlgyTEtTVkw3SyZ3aWRnZXROYW1lPXNwX3Bob25lX3NlYXJjaF9hdGYmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl

If it works for you it's more economical to diy the mixture for long term use. The upfront cost is greater (but the ingredients make more) so its better to start with a prepared version to see if you like it.There's a recipe on this subreddit.

https://www.reddit.com/r/GERD/comments/a1lpfh/how_to_make_your_own_gaviscon_advance_at_a/

Lol. you eat it. I've used it. It seems about as effective as Gaviscon Advance.

https://www.amazon.com/Reflux-Gourmet-All-Natural-Supplement-Heartburn/dp/B086ZYPWQK/ref=sr_1_3?crid=18UN3XIQXYZX4&keywords=reflux+gourmet&qid=1657123925&sprefix=reflux+%2Caps%2C140&sr=8-3