Though I ended up spending a large chunk of money, I did save quite a bit by shopping at multiple locations. If I were to do any of this again today, I would likely simply buy everything from Ultimarc. Below is where I did all of my shopping.
Ultimarc: Andy has been in business since the early days of arcade games, and the IPAC has been around since the earliest days of MAME. The IPAC lets you hook up any type of digital input to your PC.
The Real Bob Roberts: I never found out who the fake Bob Roberts wass, but the real Bob was a great guy to work with. He was just a regular guy from Louisiana who used to work in the arcade business, who then ran his own arcade parts website. He had most parts you would need to build your first control panel, with different specials every so often. He also bought in bulk from Happ Controls, so most everything he sold was actually cheaper than you or I could have bought from Happ directly. If what you needed to buy was there, I would recommend buying from him first.
Happ Controls: Happ has a highly organized and informational website. You can likely find info for almost any part you may be looking for, and you can get practically any part you need there if you cannot find it somewhere else.
Betson: This was the most fun place to shop back when I worked on these controllers. The only things I bought there were the trackball and the gearshift, but it was a great place to shop. Instead of ordering online, I went in person to their Buena Park, CA office, which also doubles as a showroom. Since they were a full on arcade supplier, they had a showroom of games when you first walked in their office, with all of the games set on FREE PLAY. I probably spent more time playing than shopping that day
Below are all links through the Internet Archive.
Video Connect: They had Ikari Warriors style LS-30 joysticks for fifteen dollars each. Also spent the extra five dollars for the LS-30 wrench that I needed for later.
Druin's Rotary Interface: Back then, you could not simply hook up an LS-30 joystick to an IPAC. To do that, you needed this interface. Today, Ultimarc sends a kit you can add to your current joystick and connect to the IPAC that can add rotary functionallity.
MAME Analog+: A version of MAME that, back then, had better analog support and better support for the LS-30 joystick. Today, MAME handles all of this just fine.
Oscar Controls: Back at that time, Oscar made the best replacement arcade spinners, along with other cool accessories for arcade control panels.
eBay: Some things I just could not find new anywhere; for those, I turned to eBay.