as others have mentioned - there are 8 lb. db avail out there. others come in 1 lb. increments - you just have to shop around. try a sporting goods store.

there are also adjustable dumbell sets where you slide the plates on and off to vary the weight - but you might not be interested in making that kind of investment.

You didn't say what exercises you are doing with these - you could try holding two (5+3) together in one hand - or use an alternate weight depending on what you are doing -

- water weighs 8 lb to the gallon - so you could work with gallons of water
- or fill a smaller water bottle with sand and weigh it on a scale
- if sand alone is too light you can add rocks or lead fishing weights or even lead shot in the middle of the sand - or even just hold the bags of lead shot.