There are lots of options out there.
Folding setup:
Fork
US $79.36 | Snow MTB Moutain 20inch Bike Fork Fat bicycle Fork Air Gas Locking Suspension Forks Aluminium Alloy For 4.0"Tire 135mm
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Headset
US $3.65 33%OFF | 44mm Aluminum CNC Folding Mountain Bike Headset Internal Sealed Bearing Folding Bike Headset Bowl Series for SP8 SP18
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Optional Spacer
US $2.84 5%OFF | 1pcs Silverock Alloy Bike Headset Spacer 2mm 3mm 5mm 12mm for Folding Bike 44mm Bicycle Headset Parts
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Folding Stem
US $35.20 | Folding bicycle Stem handlebar Stem Quick Release Bike Stem Toothless 28.6mm Folding accessories
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BMX Setup:
Fork - Same as Above
Headset
Many others will work. Needs to be threadless, 28.6mm fork dia, 44mm cups. You'll also want to be sure it has a cap, since the folding stem acts as a cap.
Optional Spacers
Amazon.com : 12 PCS Bicycle Headset Spacer Bike Handlebar Stem Spacers Threadless Aluminum Alloy Headset Stem Spacer Set Fit 1 1/8-Inch Stem For MTB BMX Mountain Road Bikes Cycling 2MM 3MM 5MM 10MM(Black) : Sports & Outdoors
www.amazon.com
These go well with the above headset. You want the ID/OD to match the headset.
Stem:
Amazon.com : Black Ops Defend`R BMX Bicycle Stem : Bike Stems And Parts : Sports & Outdoors
www.amazon.com
Or any 1 1/8 / 28.6mm stem, but need to be sure bar diameter of 22mm or 7/8. I went the route because I was concerned over bar height with a topload stem.
Bars:
The largest range of BMX Handlebars anywhere on the web! Next day delivery available on all bars at SourceBMX. Free gifts with all orders. 100% Rider owned and Run.
us.sourcebmx.com
Personal preference, I went with Jet 8.5 tall bars. They were $30.
Bar height is an important consideration when doing a BMX setup, as they are fixed and cannot be raised without purchasing extenders, alternative stems, or replacing with taller or shorter bars.
I left the fork stem uncut so, I can move the bars up or down via spacer placement.