My Mum's wheelbarrow has a knackered tyre. Years of living outside and being used without the correct amount of air in it have killed the rubber. The wheel itself is OK, but replacing both is a lot easier. We couldn't find a tyre on its own anyway.
Step 1: Oiling up
Unless you treat your barrow with the reverance normally applied to a classic car, the bolts will be rusty. Spray with penetrating oil and go and make breakfast. Spray again and go for a cup of tea. Clever people will spray and leave overnight, this stuff is good, but needs time to work.
Step 2: Unbolting
One side of the bolts has a 13mm nut, the other a pointless slot in the head. No screwdriver is going to hold the head when the presure is applied, but a set of molegrips will do the job. It's time for brute force! Actually, thanks to the oil, the bolts undid pretty well.
Step 3: Remove the axle
Engineers will say they "drift the axle out", I just whacked it with a mallet as the new wheel doesn't come with one. A shot of the penetrating oil isn't a bad idea before you start belting things, although this one wasn't a tight fit. There are two spacers lose, one either side. Don't lose them.
Step 4: Reassembly
Clean and grease all the bolts and carefully put everything back together. Then remember the spacers on the axle, take it all apart, put them in and reassemble. Make sure the U-brackets are the right way around. Grease between all the metal-to-metal joins in case you want to do this again in the future, but wipe all excess grease away as it will attract dirt.
Step 5: Pump up the tyre
Use a footpump to inflate the tyre to whatever it says on the side and then the barrow is ready for use. Buying new barrows is a hasstle (they don't fit in the car very well) and chucking an otherwise servicable tool away a waste.
The new wheel came from Torrys Hardware in Warwick. If you manage to get just a tyre, the steps are pretty much the same, you just need to remove and replace the rubber bit as well.