Yes the lead that is burned along with the fuel is a harmful poison.
oil
In the US in 1950, the average price of a gallon of regular leaded gasoline was 27 cents.
The average price of a gallon of regular leaded gasoline in 1957 was $.31 cents.
The average price of regular leaded gasoline in 1961 was .31 cents per gallon.
Nowhere I know of. You can however buy a lead additive to add to unleaded fuel, at any auto parts store.
Leaded gasoline has been illegal to sell for use in licensed street driven vehicles in the US since January 1, 1996. Leaded gasoline is restricted to two uses in the United States: Sanctioned Racing Events and Aviation...Older cars can use unleaded Gasoline but you may have to richen the mixture some to maintain the same performance and If your vehicle is older than 1970 you may have to install hardened exhaust valve seats or use a lead substitute which is added to the gas tank.. Vehicles that used leaded gasoline had a lower rate of part replacement for such things as valves,rings and pistons,also there was less wear on the cylnder walls,this was because the lead in the gasoline acted as a type of lubricant.. There are several brands of lead substitutes which are synthetic and safe to use if you have an older car that needs leaded gasoline these are available at most all parts/automotive stores..
arms and loans
In 1964, the US national average price of a gallon of regular leaded gasoline was 30 cents. That is equal to about $2.11 per gallon in 2010 dollars.
In 1978, the US national average price of a gallon of regular unleaded gasoline was 67 cents. That is equal to about $2.24 per gallon in 2010 dollars. Also, you could still buy leaded gasoline in 1978, which was 63 cents per gallon.
Since leaded fuel is no longer readily available at most all gas stations, you may have to use an additive. Proper leaded fuel is a little bit better for your engine than non-leaded additives, but hard to get and quite expensive. An additive like Tetraboost adds the lead to unleaded gasoline with lead-scavengers. All vehicles sold in the US from 1971 on are made to run on unleaded gasoline, so there is no need for an additive in your 78 unless you have an older unconverted engine in it. It should run fine on very low octane fuel.
In 1970, the US national average price of a gallon of regular gasoline was 36 cents. That is equal to about $2.02 per gallon in 2010 dollars.