Both of the 1 million peseta weapons (Chicago Typewriter and Infinite Launcher) have
infinite ammo. (The Chicago Typewriter is the same thing as a Thompson machine gun or a street sweeper without the round drum on the bottom...it's not some sort of typewriter with a barrel sticking out the back like I wondered the first time I heard the name.

)
The Hand Cannon is the only unlockable weapon that isn't infinite ammo (unless you fully power it up, then it is). You can find HC ammo, it comes in a Gold box, but is more hard to find than the Magnum shells.
I hear the Mine Launcher has ammo but I've never seen it. I guess you only see it when you have the weapon in your possession? I never buy it unless I'm not going to save and I'm just joking around.
Re: the Bow Gun, unfortunately no, it's not available outside of Separate Ways. I really did like using it a lot though...very powerful. 'Identical to an arrow with a grenade on it' is what I thought the whole time.
The other unlockable weapons...the Matilda...don't bother. Shoots 3 round bursts, uses regular pistol ammo. But it eats ammo like crazy. Just for experimentation - I once started a new round with it. Had around 500 pistol shells. I swear I went through all of 'em in the span of 2-3 minutes (at most).
The PRL, Plaga Removal Laser...well, there's a weird one. In the Ps2 version, apparently the PRL was useless. Basically people said it was little more than a gun that had the effect of a flash grenade. Not so in the Wii version! They upgraded that baby! It's friggin' awesome! It is an instant 1 hit kill (like the Hand Cannon or a rocket launcher shot) when charged (which takes a matter of a second), BUT a) it also one hit kills bosses (like Verdugo...unfrozen, which is insane because even the HC don't do that kind of damage on him) b) it automatically seeks out targets and shoots a # of beams to kill them all/you don't even have to aim it, practically, and c) it will instantly destroy all boxes, doors, barrels, etc in one shot, saving you some 'scavenging time'.
People say "well the CT/IRL/HC/PRL ruin the game because it's too easy then", I agree but disagree. I still have lots of fun playing through the game, maybe it's because I'm a huge fan though.
I can't wait for RE5 man! I had no idea if they were going to keep the over-the-shoulder view for any more RE's after 4, but that's what RE5 looks like!
On the ammo note - remember, if you run low on ammo on a weapon, you can upgrade the capacity at the merchant and it will instantly give you a ammo refill up to that capacity. Say, your current Blacktail capacity limit is 18 and you have 0 shells. If you find a merchant with the Blacktail 21 capacity upgrade and buy it, you will instantly have 21 bullets again. That came in handy for me with the Striker the very first time I played on Pro. I had like 10 shotgun bullets when I got to the merchant with the exclusive upgrade - and adding 100 (the exclusive) saved me...gave me enough ammo to finish the game with.