I have an ESP 400 Series Telecaster that I'd really like to rebuild, but I'm having trouble looking for parts. Could anyone point me in the right direction?
I guess it depends on how much you love the guitar, the fact it's an ESP though this should be fun.

grover - 18:1 mini rotomatic you'd want
schaller locking tuners perhaps

graphtech tusq nut

pickguards -
IKNMusic (ebay seller)
dragonfire guitars

custom knobs / strap locks

ebay (used are on there too if you're on a budget)
guitar shops / official distributors
you can honestly have a lot of fun with this, two humbuckers if the routings right like seymour duncans telecaster he plays.

start off with somewhere like stewmac or try the telecaster forum (a quick google no need to join) and see what they recommend. you can even put a tremolo on the tele if you'd want without any routing. It doesn't have to be fender to make the guitar good. I'll take an ESP over a fender (minus the deluxe models) any day.

electronics plate

and lastly electronics
CTS potentiometers - A250k (tele pickups) A500k (humbuckers)
0.022uf capacitor (tele single coils) or 0.047uf (humbuckes)
24 AWG wire - cloth , nickle braided or you can get shielded wire
3m of copper shielding tape for more clarty in the guitar
CRL for a 3 way switch or oaks and grigsby for a super 5 way switch
switchcraft for a quality input jack

for wire you can get shielded wire off of guitarelectronics.com , copper tape get it off of ebay. If you want push pull pots go with alpha, dimarzio or bourns. Capacitors wise I like USSR surplus of paper in oil capacitors. Expensive but not bare knuckles 14.50 (pounds) expensive.

wiring diagrams

besides that just polish the frets and do al the maintenance and it'll play and sound like better than before.