I'm trying to make a melee/explosives build, and I'm having a he'll of a time trying to find where to buy and find explosives. I would also like to know tips on maxing out the damage you can do by using explosives. Currently I have a hard time trying to kill enemies with just explosives. Thanks!

Maxing demolition expert is a very good way to skyrocket your explosive damage.

As for ammo the gun runners and boomers usually carry some, just cycle through the vendors if you need more.

Early on Chet's in Good Springd and the Mojave trading post or Nash in Primm will be your main sources of ammo, later the Boomers, BOS, Gun Runners, and Hoover dam usually have a fair amount of ammo. As for maxing damage your going to have to pump points into it at the start, try grabbing the duck and cover skill books early on, for perks demolition expert is a must have all 3 ranks that extra 60% damage really helps out. Its going to be tough for the first few levels since you low skill means foes will take multiple hits and you'll lack the caps to really stock up on ammo but around level 5 or 6 those issues should fade at that point you'll be doing good damage if you put most of your points into explosives and you'll have enough caps to keep buying ammo since its rarely found. I'd try and take out the powder gangers that'll net you enough dynamite to keep you going for a while. Your going to have to utilise every type of explosive to get by early on, lay traps with mines or powder charges and lure enemies in by taking pot shots with dynamite or a 40mm. I find the 40mm extremely useful so get on asap it'll be your best friend for a long while and even at late levels its good for low level or single foes. Mines will be you close combat and main indoor weapon, 40mm and grenade machine gun are your mid to long weapons, and the missle launcher being akin to your sniper rifle with grenades and dynamite filling short to mid ranges.

For melee I suggest having your strength up.

Cheers