I get my supplies in Divisoria. Its a place in downtown Manila, near Tutuban Mall. For the fabrics go to General Shopping Center in Ylaya St. Its a building with many shops inside that sells all kinds of fabrics; ripstop, polyester/nylon taffeta etc.. You can also find soft Tulle there for your bug net.
I just bought recently this material for bug netting which is much stronger than soft tulle, probably its no-see um but im not sure but im sure can't tear it apart. If your interested in checking this material you can find the shop that sells them on the same building, their shop is located just outside but on the other side of the building on Carmen Planas St.
And on that street just across is another building (BDO bldg) with fabric shops inside for your option. On both buildings you can also find shops that sells poly threads, zippers, reversible zippers sliders, grosgrain, cord locks d-rings etc....basically all your sewing needs.
For webbing straps just go across on Tabora St. Head down the street and you can find this small air-conditioned shop on the right side (forgot the shop's name) that sells Polypro straps, they also have tubular polypro. (no Polyester straps) They're selling it by rolls which is 150ft. around $6-$8 per roll.
While in tabora St. look out for the street vendors that sells fabrics, you might chance upon on even more cheaper price ripstop or taffeta. On the other side of Tabora St. is Juan Luna St. There are shops along the street that sells heavy waterproof fabrics for tent materials, fishing materials if you are interested, webbing straps, and upholstery stuffs like fiber fillings.
Good luck and Remember to negotiate with the prices, haggling is common there. Common prices for ripstop is $1.50/yard so don't pay more than $2 for a yard.
Also here are list of Mountaineering and outdoor shops in the Philippines just in case you need 'em.