I would avoid the foot if at all possible, for a few reasons. It is the furthest away from the heart you can get, and it will take some time for the shot to travel onwards and upwards. Also, if you end up missing, your foot is going to hurt like a motherfucker, and the potential for an abscess/infection increases greatly due to its distance from the heart. Next, the veins roll very easily so they aren't as easy to hit as you might imagine, despite them being seen easily. The veins in the foot aren't in fact round, but more so oval shaped. This makes it easier to pop thru the back of the vein and miss partially.
If you have NO veins left in your arms/wrists/hands, then you need to either - Take a break from shooting up and let your veins try and heal, or take a look at a vein map and find some other veins in your body that are easier, and safer to hit than your foot.
I know you said your options are limited, but it would be in your best interest to try and find the smallest gauge syringes as you can. Ideally 30g or 31g. I don't know where you live, but they sell them OTC this should NOT be an issue. Alternatively you can go to a needle exchange, or order them off of the internet. They are very cheap.