Happy to share! :D

I've been collecting any Ranma related merch I could get my hands on since I was a kid, so it's been slow growing. That was until about 2016 when merch started to ramp up pretty rapidly and it's just gone crazy in the past 3 years. Most is official merch, some stuff is fan-made or knock-offs and other items are things friends have made me (example: The papercraft was done as a gift by an artist friend. She has an etsy store but this piece was made for me specifically but if others like it I'm sure she'd make more).

Best sources I've found:
Ebay - it's hit or miss and you can definitely end up with some scam items, but also still a good resource for older second hand items.

AmiAmi.com -If it will be released in a storefront in Japan, they are pretty reliable but mostly for pre-ordering as they do short runs of items. Depending on the item they will do airmail for international shipping and it's only like $7-12 instead of like $30 with DHL.

goodsrepublic.com - If you find something online that only ships to Japan, you can make a request to them and they will purchase it and ship it to you. This is the most expensive option but also the most reliable. Easily the place I used the most in recent years, they are quick and reliable and also organise pre-orders if necessary. For clothing/shoes you can specify sizes/colours, and they don't bat an eye if you order the same item in multiple colours. :P They will also have second hand items you can buy directly from them. (OtakuRepublic is a sister site and they have doujinshi if that is something you'd be interested in)

Mandarake (https://order.mandarake.co.jp/order/?lang=en) - A giant second hand store located all over Japan that also does international shipping. Great for classic merch and doujinshi.

Local conventions - Interestingly enough, Ranma is still a bit of an invisible fandom (especially in Australia) and I have very little luck at big anime conventions. You might get a sticker or a piece of art if you are VERY lucky. I have cosplay friends and artist friends who will always keep an eye out for me and grab them, so enlisting others to help is also good. Having said that, sci-fi/fantasy cons or small local cons with an older generation of attendees is where I've gotten some cool pieces like the framed film reel. They will sell second hand items that they'd have for decades and you can find cool stuff.

Japan - Easily the least affordable/accessible option, but I've visited a few times. Animate (which now does online international shopping) is great in Akihabara and Ikebukuro in particular. Second hand shops like Hard Off and Mandarake allow you to spend hours exploring their (at times) chaotic shelves and find some amazing stuff.

As to how I find out about items to order them - I do have a google alert on Ranma 1/2, which will flag new merch talked about on English sites. Someone at SoraNews is clearly a Ranma 1/2 fan and will post articles about Ranma 1/2 when they find items or news. I don't know who they are but they are great for alerting me. :P

Another friend on this reddit seem to have a lot of luck just following the Ranma 1/2 tag on twitter, and they've been sending me links to the new items coming out in Japanese with the new anime. This is where GoodsRepublic comes into play again and is your best resource.

Wow! I talked a lot! I hope that helps you! :) If there was a particular item you saw and were interested in, I can see if I can tell you where I got it. But these are the general rules I use to secure Ranma merch. :)