You won't be able to find the foam version around here. Don't even ask why. It is just not here.
The cheapest would be from small family-run shops operated in Chinatown or heartland HDB areas. But you will be hard pressed to be able to find one! I'd rather buy from Watson's or Guardian or even Johor Bahru across the Causeway.
u can buy rogaine from mustafa centre or watson/guardian but its costs whooping s$95 and it will last only for one month as it is only 60g. better buy it from ebay where it will cost only s$35 including shipping.
in one of the online classifields, not sure if its gumtree.com.sg or craiglist, I found one guy near woodlands mrt selling it for the same s$35 and purchased one can from him. if u want I can pass his ph num to u. and ya its the foam version.