If you should ever be stranded (or live) in Belgium and in need of help, there is the MSOC center.
They have one or multiple in every province/ major city. Here you can get methadone, suboxone, free "injection kits" with syringe, citric acid and a sterile cooker. Also sterile filters I believe.
I'm not sure if the center is called MSOC in the French speaking part of the country though, I will do some research on this and also try to update this with phone numbers for all major cities in Belgium.
Should you need help I am pretty sure any of these offices can help you find your local help center if you're in a place not listed below. This should be pretty complete for the Flemish part of Belgium.
So far I found:
Oostende, West-Flanders province:
Perronstraat 1, Oostende
tel.
059/51.63.29, 059/51.63.09
fax
059/51.76.83
Ghent, East Flanders province:
Gewad 13, 9000 Gent
09 223 46 44
Antwerp, Antwerp province:
Schijnpoortweg 14
2060 Antwerpen
t 03 201 12 60
f 03 201 12 99
Limburg province:
Administration headquarters: Salvatorstraat 25, 3500 Hasselt
MSOC Limburg: Welzijnscampus 7 bus 12, 3600 Genk
Regional offices
Beringen: Laan op Vurten 57, 3581 Beverlo-beringen, 011/45 10 69
Genk MSOC: Welzijnscampus 7/12, 3600 Genk, 011/71 60 00
Hasselt MSOC: Salvatorstraat 25, 3500 Hasselt, 011/87 44 07
Hasselt CGG: Salvatorstraat 25, 3500 Hasselt, 011/27 42 98
Hasselt Preventie: Salvatorstraat 25, 3500 Hasselt, 011/71 60 01
Heusden-Zolder: St-Willibrordusplein 4, 3550 Heusden-Zolder, 011/45 61 57
Lanaken: Pastorijstraat 7/2, 3620 Lanaken, 089/71 16 04
Maasmechelen: Oude baan 207, 3630 Maasmechelen, 011/71 60 02
Neerpelt: Stationstraat 76, 3910 Neerpelt, 011/64 78 11
Sint-Truiden: Halmaalweg 2, 3800 Sint-Truiden, 011/67 30 40
Tongeren: Henisstraat 7, 3700 Tongeren, 011/71 60 03
Herentals: Hofkwartier 23, 2200 Herentals, 014/22 66 50
Mol: Markt 54, 2400 Mol, 014/22 66 50
Turnhout: Parklaan 162, 2300 Turnhout, 014/22 66 50