There are plenty of commercial sellers and shops over here which deal with classic gaming but there are almost no brick-and-mortar ones left, 99% of that stuff gets sold online. Mind you, there are some shops in malls with current stuff, the occasional pawnshop with a few games or flea markets with the usual dirty smelly cart-/disc-only stuff, can be worthwhile to check those places out if there's an opportunity.
Oh, and don't forget that the vast majority of games you can find here will be PAL/50Hz, so if you don't have an option to switch the games to 60Hz, I wouldnt bother with them if I were you. Some 8bit and 16bit cart games of various consoles (SMS, NES, MD, etc.) will run in 60Hz if your console has a 50/60Hz and/or region switch, others don't work because they were specifically made with 50Hz in mind so they will flicker or not work at all if you switch to 60Hz.
Some PAL games from later concole generations sometimes offer 50/60Hz switch options, that specially goes for DC games. It's also in some PS2 titles but sadly it wasn't a standard by any means so buying PAL games here with the intention to run them on your US systems can be a gamble.
The thing is, I bet you won't find any Neo stuff, the console was almost unheard of back then and even more so now so you'll only find the usual ebay and ebay classified ads listings. Of course the 50/60Hz PAL/NTSC problem does not apply here because Neo games were region-free.
There are some arcades in larger cities such as Munich, Berlin, Stuttgart, etc., they're usually not very well-stocked with Neo games, tho. Most of the classic arcade places have been changed to gambling dens a long time ago.