Your best bet is like what one of the previous users said is getting them from Japan using a middleman. Spyro 2 I say would probably be the hardest(like you said), I've tried looking around and most of the time you get the Japanese strategy guide coming up on Yahoo Japan auctions.I myself got a copy of Spyro 2 back in 2010 off Yahoo auctions,was about 700 yen(price and auction domestic shipping) and it cost me 1350yen(commission added on so price+ domestic shipping+ commission+ International shipping= 1350 yen) in total around £9.26 when I used TreasureJapan.My copy did not have the manual or cover art,but I must say Spyro 2 is certainly interesting in Japanese,especially the cutscenes before levels is interesting(Love Spyro's Japanese voice very cute.)
Anyway the best place to find Spyro 2 would be Amazon Japan where its anywhere from 970 yen(intotal) upwards used.
The same goes for the GBA games where you can get them used from around 341 yen upwards.
The Japanese version of Spyro fusion I have seen has been from 240-300 yen on Yahoo auctions.Spyro Season of Ice is known as Spyro advance in Japan and I've seen it on Amazon,no picture is shown but it is on the 2nd page of Amazon jp when searching for Spyro(スパイロ).
I'm not sure if you want the manuals boxes or not for the GBA games but most on Amazon come as cartridge only.
Japanese titles to search for games are : Spyro 1:スパイロ・ザ・ドラゴン (Like you said the easiest to find around 499 yen upwards.I got mine from ebay though in 2010 for about £10-15 complete)
Spyro 2: スパイロXスパークス トンでもツアーズ
Spyro season of Ice/ Advance: スパイロ アドバンス
Spyro Fusion : スパイロ アドバンス わくわくトモダチ大作戦! (google translates as Spyro Advance exciting! Tomodachi Daisakusen)
Would recommend searching Mbok auctions,Rakuten, etc as well it can simply be a matter of luck.The middleman I have used and best for me were TreasureJapan(My first and easy to use) and JapanauctionAgency(Known as Kairai on Livejournal).
From what I have heard FromJapan are meant to be pretty good to use as well.
Anyway talking of Japanese Spyro adverts here is one for Spyro 1: Lets teach kids to glide! lol