Sounds like a great trip, and looks like a nice party!
Recordshops in Amsterdam that might be worth paying a visit in my opinion are:
-Rush Hour
Spuistraat 98
New stuff + second hand, house, electro, disco, wave etc . Nice shop, not too big. In my opinion best place to go for interesting new records, but they have nice 2nd hand stuff as well.
-Wax Well
2nd hand, hip hop, soul, disco, funk, rock, nice shop not always too cheap but mostly not over the top either I think. They mostly have a nice stock. They do have cheapo stuff that can be nice too.
-Record Friend
Sint Antoniesbreestraat 64
New and 2nd hand. Rock, pop, punk wave, mostly not that much disco, but you'll never know,. Some stuff is pretty overpriced especially everything that's considered dark/minimal wave and/or industrial, though I found some bargains there as well, sometimes they put stuff in the cheapo bin if it hasn't sold for some time. It happened that I bought someting from the euro bin, which was also in another crate for 11 euro. For example I once found DF & Pam - On The Beat and Midnight Passion - I need Love (ZYX) in the eurobins here.
-Second life Music
Prinsengracht 366
Rock, pop all kinds of everything, disco section is not that interesting, but there might be something interesting in their "80's" crates.
-Distortion
New + 2nd hand, pop, rock punk electro soul, disco hip hop etc
Nice, but there are so much records that there are huge piles of records, it makes browsing not that easy.
http://www.distortion.nl/
Westerstraat 244
-Record Mania
2nd hand, Rock/pop, soul, disco, punk and more
Ferdinand Bolstraat 30
-Record Palace
2nd hand, mostly Rock/Pop but not only, it's very close to the Paradiso club. Not my favourite, but the fact remains that you can can find stuff in unlikely places. At least they sell recors so you'll never know!
Weteringschans 33
Amstel Antique
2nd hand records in several styles, rock/pop disco, soul, metal, punk
Can be interesting at times.
Amstel 110
You can see the locations here:
http://www.recordshops.org/
Furthermore:
The market on the Waterlooplein can be interesting at times as well. It's a bit like a fleamarket but also with professional sellers selling boring 'funny' T-shirt some tourists like, but there are also record seller and every know and then they have you can find interesting stuff. Thing is not every seller is there each and every day, but it's very close to the Record Friend listed above so it's always worth paying a visit in my opinion.