In the weapon store under the multiplayer option, you need to select the weapon screen where you can purchase all weapons. The total cost of all weapons is 26,000 credits. The easiest way I've found to gain credits quickly is by playing golden gun since if you finish with the top score, you'll make 1,000 credits per match. Good luck!
the following credit guide has been made by user SemtexDrinker:
Golden Gun : 10 credits / point
Classic / Territory Control / (Team) Conflict : 40 credits / point (this is your individual score, not team)
Bond Versus/Evasion : As Organization, 125 credits / kill, as Bond, 125 credits / kill, 250 for escape, 125 for each bomb