On my overland trip to Thailand, I was planning to get Visas on Arrival for both Laos and Thailand.

However, I've just read that the overland Thailand visa will only grant me 15 days, not enough. Hence my questions (considering my nationality is Belgian):

A. How can I get a 30-day or longer visa if I'm planning to enter overland? Does that require me to go to the consulate?

A1. Where is that?
A2. How much time and money should I anticipate?
A3. Which documents should I prepare?

B. If I enter Laos overland, how many days VOA will I get?
B1. How much money do I need for this?
B2. Which documents should I prepare?

I know I could search the internet for this, but there are so many factors influencing this (policies change over time, nationalities differ, points of entry differ ...) that I'd rather ask people that live close by.

A. yes. i hear that, strictly speaking, the amount of time given to you upon entry is at the border guard's discretion. so, in theory, you could ask nicely at the border for more days. i'd not bank on it though.

A1. Next to the Kunming Hotel. The consulate is listed on this site.
A2. apply between 9am and noon. get back two days later, if i remember rightly. might even be next day. i paid CNY230 for a 60 day (EU passport). this is issued with an 'enter before' date that's a few days after you say you plan to enter the kingdom.
A3. passport, two copies of application form, each requiring photo, passport copy, plus another simple summary form

B. 30 days
B1. US$37. payment in other currencies is permitted, it appears from a printed list that i snuck a look at, but it works out a bit more expensive. i'm pretty sure CNY is possible, but to be sure, i'd show up with the greenbacks.
B2. passport, photo (maybe 2, i forget)

this information is current as of early November for the Thai consulate in Kunming and November 30th for the Laos border crossing at Mohan-Boten.

Thanks hedgepig for the comprehensive answer! :D