Once you've figured out exactly what you want, the next stage is to find the best deals for the keyboard you want. Fortunately, there are plenty of places you can visit to find sales of Model F and Model M (or even the wider IBM and family) keyboards. This page will outline and describe some of the best places you can visit.
eBay is by far the main place vintage keyboards get traded on, so you're very likely to find multiple Model Fs and Ms for sale on there. Unfortunately, pricing on eBay as of late is at a premium but good deals can still be found if you're vigilant and willing to play the waiting game.
There are two main strategies you can employ:
eBay is not a perfect platform however as items and their condition can be misrepresented by sellers. To make matters worse, eBay has incorrectly ruled terms like "clicky" in the context of keyboards to be a subjective term in the past, effectively allowing sellers to apply the term "clicky" without the keyboard necessarily having clicky switches (which the Model F or M you're after likely has). Thus, it's always recommended that you do basic research/"homework" on stuff you find such as searching part or model numbers the keyboard you're looking at has to see if it matches known examples found in the past. You can use the Keyboard Part Number Database or simply Google the numbers to help with this.
Today, there are many places you can visit to find people's keyboard sales listings outside of major platforms like eBay. Before checking these out, in case you're unfamiliar with terminology:
r/MechMarket is the primary subreddit for selling and buying keyboards and probably the biggest keyboard trading space outside of eBay. Whilst overall it tends to have more modern keyboards and parts being traded, Model Fs and Ms (and other vintage IBM keyboards) do show up with frequency. Business is typically done through PayPal with business protection enabled, so you can fall back on PayPal in case you don't get what you paid for. It has a strong reputation system and is well moderated with a lot of rules to follow that you can see here.
The Official r/ModelM subreddit has its own Discord server with space reserved for user-to-user trading and posting prospective deals found on eBay. You can join the subreddit here.
The largest vintage keyboard community forum. Looking or posting here is very likely to find or attract the people you're looking for. It has dedicated space for keyboard trading, although you'll need a forum account to access it.
Unicomp manufactures and sells brand new Model M buckling springs keyboards. As you may/may not be aware, they're a company formed by former IBM/Lexmark employees that brought tooling and IP needed to continue producing Model Ms after Lexmark and IBM UK discontinued their keyboard production in 1996 and 2000 respectively. Unicomp produces several models that can either serve as a continuation of vintage designs like the Unicomp Classic, Ultra Classic or PC 122 or their own updated designs like the New Model M and Mini Model M. There are both pros and cons with buying a Unicomp over a vintage Model M, a topic covered in the Should I get an old or new Model M? section of the Buyer's Guide.
Brandon (aka, ClickyKeyboards) is a long-time professional bolt modder and Model M reseller. Whilst some may consider his prices to be on the step side, Brandon has a good reputation for doing the job well and providing support. If you're after a vintage Model M and don't want to worry about problems such as broken rivets or dirt on arrival, ClickyKeyboards is a good option.
Model F Labs is a project set up by deskthority user Ellipse in 2015 as a group buy that funds and produces brand-new Model F keyboards based on the IBM 4704 Model 200 (aka "F62" or kishsaver) and Model 300 ("F77") capacitive buckling springs keyboards. As of 2021 Q4, they're still accepting new orders and a final round of the group buy is due in 2022. In addition to the keyboards, Ellipse also sells a plethora of keycaps and spare parts like springs and foam. If you want a brand new Model F experience, there is practically no other option.
Events like yard, garage or car-boot sales or brick-and-mortar thrift stores are also possible ways to find keyboards. The major benefit of putting in the effort of going to these is that you can try them before you buy. You can check the key feel, the clickiness or see the part number clearly to verify it's the Model F or M you want.
You can buy keyboards from classic auction/trading websites, specific forums and Discord servers and even in person, with the general idea being you should be checking all of the above regularly to find the best deals. Unfortunately, there will be keyboards that are practically so rare that they're 'unicorns' and you simply won't find one quickly or cheaply. But certainly, these places should be checked out if you're simply after the general/standard Model F or Model M keyboard. If you need any additional advice or clarification, feel free to get in contact with me or make a post on r/ModelM!