Fishing weights?? cheap and you can get them at any hardware store.. how about old tires they are balanced with lead...
Fishing weights are pure lead they work great for black powder loads but are a bit soft for anything other than standard 38 special loads or smaller calibers with a similar MV range. The reason you can't use plain base pure lead for higher velocity pistol and rifle rds. is because the pure lead isn't strong enough to withstand the pressure the higher velocity rds. generate without melting the bullet base,unless you use a gas check to protect the base of the bullet,gas check bullets generally don't come into play until you reach MV of 1400 fps. with harder cast bullets or you using a softer alloy that can't withstand the PSI your load is generating but you still want the bullet to expand.
A couple thing you have to take into consideration with cast bullets. First the bullet has to fit the bore of your firearm,you can get by with .001" over the groove to groove dia. but .002" is better some firearms can even handle .003" if they have large or warn throats like some military surplus rifles. The lead alloy bullet will compress/squeeze down easily to fit the bore,over sized copper jacket bullets on the other hand generate more pressure when there over sized because there about 15 times harder than hard cast lead bullets.
Once you know what size bullet you need the second thing you need to know is what alloy your using and what it's hardness/BHN is,this will tell you the Min. PSI required to swell/obturate the base of the bullet so that it seals the bore,without sealing the bore properly you get several things gas cutting around the bullet which causes leading and lower MV because the burning gases escapes around the bullet instead of pushing it forward.
Lyman reloading and Lyman cast bullet manuals tell you what PSI there Min. & Max. loads generate so it's just a matter of taking your bullets BHN multiplying it by 1422 and that will give you the Min. PSI required to obturate the bullets base and seal the bore. Cast bullets with a hardness or BHN of from 10 to 12 rang work perfectly fine for loads around 1200 to 1300 fps no matter what you shoot them in after that point you either need to used a higher BHN bullet or us a gas check.
For my full house 357 Mag. load I shoot a Lee tumble lube flat base cast bullet made from wheel weights and water dropped which doubles the bullets hardness to around 18 BHN MV is around 1400 fps. for my rifle bullets most of which I shoot between 1500 and 2000 fps. I use the same alloy but apply a gas check because the rifle bullet simply get driven faster plus I like to leave my cast hunting bullets relatively soft so the expand to a degree although the flat nose designs I use do a good bit of damage on there own.
Some good sources of lead are tire shops,car dealerships,scrap yards,ebay,Cast Boolits Swapping & Selling section,Plumber,people that work in construction,sailboat keels,shooting ranges just to name a few.
Commercial cast bullet alloys.
http://www.rotometals.com/Bullet-Casting-Alloys-s/5.htm
For all you guys working with pure lead Rotometals has an alloy called Super Hard
http://www.rotometals.com/product-p/30_antimony_70_lead.htm using 15 lbs. of pure lead or stick on WW and 2 lbs of Super Hard will get you an air cooled BHN of around 12 after two days setting water quenching or oven heat treating will harden it even more.
Good articles for bullet casters.
http://www.lasc.us/CastBulletNotes.htm
http://www.lasc.us/IndexBrennan.htm
http://www.lasc.us/SuperHard.htm